Positive electrode active material, positive electrode, nonaqueous electrolyte cell, and method of preparing positive electrode active material

ABSTRACT

Disclosed herein is a positive electrode active material prepared by mixing a lithium-containing compound, a compound containing a transition metal to be put into a solid solution, and a compound containing a metallic element M2 different from the transition metal, and firing the mixture to form composite oxide particles, depositing a compound containing at least one element selected from among sulfur (S), phosphorus (P) and fluorine (F) on surfaces of the particles, and firing the particles, whereby each of the particles is provided with a concentration gradient such that the concentration of the metallic element M2 increases from the center toward the surface of the particle, and at least one element selected from among (S), (P) and (F) is made present in the form of being aggregated at the surfaces of the composite oxide particles.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/878,497, filed on Sep. 9, 2010, which claims priority to JapanesePriority Patent Applications JP2009-208505, JP2009-263301,JP2010-105024, and JP2010-105025 filed in the Japan Patent Office onSep. 9, 2009, Nov. 18, 2009, Apr. 30, 2010, and Apr. 30, 2010,respectively, the entire content of which is hereby incorporated byreference.

BACKGROUND

The present application relates to a positive electrode active material,a positive electrode, a nonaqueous electrolyte cell, and a method ofpreparing a positive electrode active material. More particularly, theapplication relates to a positive electrode active material, a positiveelectrode, a nonaqueous electrolyte cell, and a method of preparing apositive electrode active material by which to realize a nonaqueouselectrolyte cell having high performance and showing little capacitydeterioration when subjected to charging and discharging inhigh-temperature environments. In detail, the present applicationrelates to a positive electrode active material includinglithium-transition metal composite oxide.

Attendant on spreading of portable apparatuses such as video cameras andnotebook type personal computers, in recent years, there has been anincreasing demand for small-sized high-capacity secondary cells andbatteries. The secondary cells in use at present include thenickel-cadmium cell and the nickel-hydrogen cell, which use an alkalineelectrolyte. These secondary cells, however, are disadvantageous in thatthe cell voltage is as low as about 1.2 V and it is difficult to enhanceenergy density. In view of this, nowadays, the lithium ion secondarycell which is higher in voltage than other cell systems and high inenergy density has come into widespread use.

Due to its higher charged voltage as compared with other cell systems,however, the lithium ion secondary cell has a problem in that when usedin such mode as to be left for a long time in a charged state, itscapacity would be deteriorated and its useful life would be shortened.In addition, when the lithium ion secondary cell is used underhigh-temperature environmental conditions, a rise in internal resistancewould proceed, making it very difficult to secure a sufficient capacity.Solution to these problems is keenly requested.

LiCoO₂, LiNiO₂ and other lithium-transition metal composite oxideparticles are widely used as positive electrode active materials ofLithium-ion secondary batteries. Lately, various arts to improve thestate of the particle surfaces to obtain better properties of thelithium-transition metal composite oxide particles by forming coatinglayer at the particle surfaces or diffusing some material from theparticle surfaces have been proposed.

For example, in Japanese Patent No. 3197763 (hereinafter referred to asPatent Document 1), there is shown a method in which a metallic salt orhydroxide is added to a positive electrode. Besides, Japanese PatentLaid-open No. Hei 5-47383 (hereinafter referred to as Patent Document 2)shows a technology wherein a surface of lithium cobaltate (LiCoO₂) iscoated with phosphorus (P). Japanese Patent No. 3172388 (hereinafterreferred to as Patent Document 3) and Japanese Patent No. 3691279(herein after referred to as Patent Document 4) shows a method in whicha positive electrode active material or a surface of a positiveelectrode is coated with a metallic oxide.

Japanese Patent Laid-open No. Hei 7-235292 (hereinafter referred to asPatent Document 5), Japanese Patent Laid-open No. 2000-149950(hereinafter referred to as Patent Document 6), Japanese PatentLaid-open No. 2000-156227 (hereinafter referred to as Patent Document7), Japanese Patent Laid-open No. 2000-164214 (hereinafter referred toas Patent Document 8), Japanese Patent Laid-open No. 2000-195517(hereinafter referred to as Patent Document 9), Japanese PatentLaid-open No. 2001-196063 (hereinafter referred to as Patent Document10), Japanese Patent Laid-open No. 2002-231227 (hereinafter referred toas Patent Document 11), and the like show a method in which surfaces ofparticles are uniformly coated with a lithium-transition metal compositeoxide and a method in which the composite oxide is diffused from thesurfaces of particles. In addition, Japanese Patent Laid-open No.2001-256979 (hereinafter referred to as Patent Document 12) shows apositive electrode active material in which lumps of a metallic oxideare deposited onto a metallic oxide layer. Japanese Patent Laid-open No.2002-164053 (hereinafter referred to as Patent Document 13) shows apositive electrode active material in which at least one surfacetreatment layer containing at least two coating elements is formed onsurfaces of cores which contain a lithium compound.

Japanese Patent No. 3157413 (hereinafter referred to as Patent Document14) discloses a positive electrode active material in which a coatingincluding a metallic fluoride is provided on surfaces of particles, andJapanese Patent No. 3141858 (hereinafter referred to as Patent Document15) shows a coating which includes a crystalline metallic fluoride. Inaddition, Japanese Patent Laid-open No. 2003-221235 describes that theXPS (X-ray Photoelectron Spectroscopy) energy of fluorine on thesurfaces of particles is specified. When the present inventors preparedthe positive electrode active material by the method of mixing ametallic fluoride and heat-treating the mixture according to thedisclosure, a practical effect on high-temperature preservability wasobserved, but the effect was limited to the effect on the surfaces ofthe particles and was insufficient on the basis of practical useperformance. Further, U.S. Pat. No. 7,364,793 (hereinafter referred toas Patent Document 16) discloses a material obtained by a method inwhich a compound having high affinity for lithium and capable ofsupplying cations is brought into reaction with a lithium-transitionmetal composite oxide.

SUMMARY

However, according to the addition of an metallic salt or hydroxide to alithium-transition metal oxide having a normal uniform form as in themethod of Patent Document 1, the resistance of the electrode increasedand it was very difficult to obtain a sufficient capacity. In the methodof Patent Document 2, the lowering in capacity due to the coating was solarge that the positive electrode active material was unsatisfactory forpractical use. The method of Patent Documents 3 and 4 was unsatisfactoryas a technique for enhancing cell performance in high-temperatureenvironments, if use is made of only the coating element, coatingmethod, and coating form disclosed in the document. Besides, it wasfound that increasing the coating amount in order to obtain a sufficienteffect leads to hindrance of diffusion of lithium ions, making it verydifficult to obtain a sufficient capacity at charge-discharge currentvalues in practical-use ranges. Thus, the method was foundunsatisfactory.

The method disclosed in Patent Document 4 to 9 was found unsatisfactoryfor enhancing cycle characteristics to a high extent and suppressing arise in resistance during high-temperature use, though a high capacitycan be maintained by the method. When a positive electrode activematerial was prepared by the method and structure disclosed in PatentDocument 12, it was very difficult to obtain a sufficientcharge-discharge efficiency, and the capacity was largely lowered. Inthe method of Patent Document 13, the effect owing to the method waslimited if the surface treatment alone was adopted. Besides, when thepositive electrode active material was actually prepared by the methoddisclosed in the document, a uniform multiple layer was formed, and theeffect was not recognized, particularly, on a rise in resistance duringhigh-temperature use.

As for the approach according to Patent Document 15, mere coating with ametallic fluoride being low in electronic conductivity and lithium ionconductivity resulted in a conspicuous lowering in charge-dischargeperformance, and its effect on charge-discharge characteristics inhigh-temperature environments was insufficient. When the presentinventors prepared the positive electrode active material by the methoddisclosed in Patent Document 16, nonuniformity or slip-off of thematerial added as the coating material occurred, and inactive compoundssuch as oxides and lithium fluoride were produced, so that the coatingfunction could not be exhibited sufficiently. In addition, it was verydifficult to obtain charge-discharge performance on a practical-uselevel, because the migration of lithium ions at the charge-dischargetimes was hampered at the solid-liquid interface. Besides, there wasalso observed a tendency of lowering in capacity, because lithium wasdeprived from the lithium-transition metal composite oxide. Thus, thematerial according to the document was unsatisfactory.

Thus, there is a need for a positive electrode active material which ishigh in capacity, excellent in charge-discharge cycle characteristicsand shows little deterioration when used in high-temperatureenvironments, for a positive electrode and a nonaqueous electrolytesecondary cell using such a positive electrode active material, and fora method of preparing such a positive electrode active material.

According to an embodiment, there is provided a positive electrodeactive material prepared by:

mixing a lithium-containing compound, a compound containing a transitionmetal to be put into a solid solution, and a compound containing ametallic element M2 different from the transition metal, and firing themixture to form composite oxide particles;

depositing a compound containing at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) on surfaces of thecomposite oxide particles; and

firing the composite oxide particles with the compound containing atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) deposited thereon;

whereby each of the composite oxide particles is provided with aconcentration gradient such that the concentration of the metallicelement M2 increases from the center toward the surface of the compositeoxide particle, and

at least one element selected from among sulfur (S), phosphorus (P) andfluorine (F) is made present in the form of being aggregated at thesurfaces of the composite oxide particles.

According to another embodiment, there is provided a positive electrodeincluding a positive electrode material which is prepared by:

mixing a lithium-containing compound, a compound containing a transitionmetal to be put into a solid solution, and a compound containing ametallic element M2 different from the transition metal, and firing themixture to form composite oxide particles;

depositing a compound containing at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) on surfaces of thecomposite oxide particles; and

firing the composite oxide particles with the compound containing atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) deposited thereon;

whereby each of the composite oxide particles is provided with aconcentration gradient such that the concentration of the metallicelement M2 increases from the center toward the surface of the compositeoxide particle, and

at least one element selected from among sulfur (S), phosphorus (P) andfluorine (F) is made present in the form of being aggregated at thesurfaces of the composite oxide particles.

According to a further embodiment, there is provided a nonaqueouselectrolyte cell including a positive electrode, a negative electrode,and an electrolyte,

wherein the positive electrode includes a positive electrode activematerial which is prepared by comprising:

mixing a lithium-containing compound, a compound containing a transitionmetal to be put into a solid solution, and a compound containing ametallic element M2 different from the transition metal, and firing themixture to form composite oxide particles;

depositing a compound containing at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) on surfaces of thecomposite oxide particles; and

firing the composite oxide particles with the compound containing atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) deposited thereon;

whereby each of the composite oxide particles is provided with aconcentration gradient such that the concentration of the metallicelement M2 increases from the center toward the surface of the compositeoxide particle, and

at least one element selected from among sulfur (S), phosphorus (P) andfluorine (F) is made present in the form of being aggregated at thesurfaces of the composite oxide particles.

The compound containing at least one element selected from among sulfur(S), phosphorus (P) and fluorine (F) of the positive electrode activematerial, or the pyrolyzed, i.e., thermally decomposed product of thecompound of the positive active material, of the positive electrode, andof the nonaqueous electrolyte cell of the present embodiment haspreferably melting point of 70° C. or more and of 600° C. or less, andalso has preferably the average diameter of 30 μm or less.

According to yet another embodiment, there is provided a method ofpreparing a positive electrode active material including:

mixing a lithium-containing compound, a compound containing a transitionmetal to be put into a solid solution, and a compound containing ametallic element M2 different from the transition metal, and firing themixture to form composite oxide particles;

depositing a compound containing at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) on surfaces of thecomposite oxide particles; and

firing the composite oxide particles with the compound containing atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) deposited thereon;

whereby each of the composite oxide particles is provided with aconcentration gradient such that the concentration of the metallicelement M2 increases from the center toward the surface of the compositeoxide particle, and

at least one element selected from among sulfur (S), phosphorus (P) andfluorine (F) is made present in the form of being aggregated at thesurfaces of the composite oxide particles.

The compound containing at least one element selected from among sulfur(S), phosphorus (P) and fluorine (F) deposited on surfaces of thecomposite oxide particles is preferably melted or thermally decomposedand melted to exist uniformly on the surface of the composite oxideparticles. It is also preferred that on the surface of the compositeoxide particles cation of the compound containing at least one elementselected from among sulfur (S), phosphorus (P) and fluorine (F)deposited on surfaces of the composite oxide particles is eliminated andanion of the compound is reacted with elements contained in thecomposite oxide particles.

Further, according to another embodiment, there is provided a positiveelectrode active material including lithium-transition metal compositeoxide particles containing lithium, a principal transition metal M1, anda metallic element M2 different from the principal transition metal M1,the metallic element M2 having a concentration gradient of the metallicelement M2 from the center toward the surface of each particle, whereinmolar fraction r(%) satisfies the formula 0.20≦r≦0.80 in the range wherethe ratio d(%) from the surface to a certain depth satisfies0.020≦d≦0.050, wherein ratio d(%)=[(mass of the principal transitionmetal M1)+(mass of metallic element M2)]/(mass of whole particles), andwherein molar fraction r=(mass of metallic element M2)/[(mass ofprincipal transition metal M1)+(mass of metallic element M2)].

According to even another embodiment, there is provided a positiveelectrode active material including lithium-transition metal compositeoxide particles containing lithium, a principal transition metal M1, anda metallic element M2 different from the principal transition metal M1,the metallic element M2 having a concentration gradient of the metallicelement M2 from the center toward the surface of each particle, whereinmolar fraction r(%) satisfies the formula 0.20≦r≦0.80 in the range wherethe ratio d(%) from the surface to a certain depth satisfies0.020≦d≦0.050, where ratio d(%)=[(mass of the principal transition metalM1+(mass of metallic element M2)]/(mass of whole particles), and wheremolar fraction r=(mass of metallic element M2)/[(mass of principaltransition metal M1)+(mass of metallic element M2)].

According to even another embodiment, there is provided a nonaqueouselectrolyte cell including a positive electrode, a negative electrode,and an electrolyte, positive electrode including a positive electrodeactive material including lithium-transition metal composite oxideparticles containing lithium, a principal transition metal M1, and ametallic element M2 different from the principal transition metal M1,the metallic element M2 having a concentration gradient of the metallicelement M2 from the center toward the surface of each particle, whereinmolar fraction r(%) satisfies the formula 0.20≦r≦0.80 in the range wherethe ratio d(%) from the surface to a certain depth satisfies0.020≦d≦0.050, where ratio d(%)=[(mass of the principal transition metalM1)+(mass of metallic element M2)]/(mass of whole particles), and wheremolar fraction r=(mass of metallic element M2)/[(mass of principaltransition metal M1)+(mass of metallic element M2)].

According to the present application, reaction at the positive electrodeactive material—electrolytic solution boundary is suppressed bycontrolling molar fraction r(%) to satisfy the formula 0.20≦r≦0.80 inthe range where the ratio d(%) from the surface to a certain depthsatisfies 0.020≦d≦0.050.

In the present application, each of composite oxide particles isprovided with a concentration gradient such that the concentration of ametallic element M2 increases from the center toward the surface of thecomposite oxide particle, and at least one element selected from amongsulfur (S), phosphorus (P) and fluorine (F) is present at surfaces ofthe composite oxide particles in the form of being aggregated on thesurfaces of the composite oxide particles. Therefore, stabilization ofthe positive electrode active material and stabilization at theinterfaces can be promised.

According to the present application, it is possible to realize a cellwhich is high in capacity, is excellent in charge-discharge cyclecharacteristics and shows little deterioration when used inhigh-temperature environments.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view showing a configuration example of anonaqueous electrolyte cell according to an embodiment of theapplication;

FIG. 2 is a sectional view, taken along line II-II of FIG. 1, of a woundelectrode body shown in FIG. 1;

FIG. 3 is a sectional view showing a configuration example of anonaqueous electrolyte cell according to an embodiment;

FIG. 4 is a sectional view showing, in an enlarged form, a part of awound electrode body shown in FIG. 3; and

FIG. 5 is a sectional view showing a configuration example of anonaqueous electrolyte cell according to an embodiment.

DETAILED DESCRIPTION

The present application will be described below with reference to thedrawings according to an embodiment.

1. First embodiment (First example of nonaqueous electrolyte cell)

2. Second embodiment (Second example of nonaqueous electrolyte cell)

3. Third embodiment (Third example of nonaqueous electrolyte cell)

4. Fourth embodiment (Fourth example of nonaqueous electrolyte cell)

5. Fifth embodiment (Fifth example of nonaqueous electrolyte cell)

6. Other embodiment (Modification)

[Gist of the Present Invention]

Lithium-containing transition metal oxides such as lithium cobaltate(LiCoO₂) and lithium nickelate (LiNiO₂) are widely used as a positiveelectrode active material in lithium ion secondary cells. However, theyhave a problem as to stability in their charged state. Particularly, dueto a rise in reactivity at the interface between the positive electrodeactive material and the electrolyte, the transition metal componentwould be eluted from the positive electrode, leading to deterioration ofthe active material or deposition of the eluted metal on the negativeelectrode side. As a result, occlusion and release of lithium (Li) wouldbe hampered.

In addition, such a positive electrode active material asabove-mentioned is considered to accelerate a decomposition reaction ofthe electrolyte at the interface, causing formation of a coating on theelectrode surface or generation of a gas, which leads to deteriorationof cell characteristics. Meanwhile, by conducting charging in such amanner as to attain a maximum charged voltage of at least 4.20 V,preferably at least 4.35 V, more preferably at least 4.40 V, in thecondition where the positive electrode-negative electrode ratio isdesigned appropriately, it is possible to enhance energy density of thecell at the time of charging. It has been made clear, however, thatwhere the charging voltage is raised and charge-discharge cycles arerepeated in a high charged voltage condition of 4.25 V or higher, theabove-mentioned deterioration of the active material or the electrolyteis accelerated, leading to a lowering in the charge-discharge cycle lifeor to deterioration of performance after storage at high temperature.

In view of this, the present inventors made intensive and extensiveinvestigations. As a result of their investigations, they have found outthat where a lithium-transition metal composite oxide with improvedparticle surfaces is used, the presence of a metallic compound on theparticle surfaces produces a high synergistic effect or a novel effecton enhancement of cell characteristics. The present invention, which hasbeen made based on the finding, aims at providing a positive electrodeactive material for lithium ion secondary cells which greatly enhancescharacteristics and reliability of the cells.

1. First Embodiment First Example of Nonaqueous Electrolyte Cell

FIG. 1 is a perspective view showing a configuration example of anonaqueous electrolyte cell according to a first embodiment. Thenonaqueous electrolyte cell is, for example, a nonaqueous electrolytesecondary cell. The nonaqueous electrolyte cell, flat in overall shape,has a configuration in which a wound electrode body 10 fitted with apositive electrode lead 11 and a negative electrode lead 12 isaccommodated in the inside of film formed casing members 1.

The positive lead 11 and the negative lead 12 are each, for example,rectangular plate-like in shape, and they are lead out from the insidetoward the outside of the casing members 1 in the same direction, forexample. The positive electrode lead 11 is made of a metallic materialsuch as aluminum (Al), for example, and the negative electrode lead 12is made of a metallic material such as nickel (Ni), for example.

The casing members 1 are each composed, for example, of a laminate filmhaving a structure in which an insulating layer, a metallic layer and anoutermost layer are stacked in this order and adhered to one another bylamination or the like. The casing members 1 are, for example, soconfigured that the insulating layer side is set on the inner side, andeach pair of outer edge portions are secured to each other by fusing orby use of an adhesive.

The insulating layer is composed, for example, of a polyolefin resinsuch as polyethylene, polypropylene, modified polyethylene, modifiedpolypropylene, and their copolymers. Such a polyolefin resin promises alow water permeability and is excellent in gas-tightness. The metalliclayer is composed of a foil-shaped or sheet-shaped member of aluminum,stainless steel, nickel, iron or the like. The outermost layer may, forexample, be composed of a similar resin to that of the insulating layer,or be composed of nylon or the like. Such a material promises a highstrength against rupture or piercing. The casing member 1 may furtherhave other layer than the above-mentioned insulating layer, metalliclayer, and outermost layer.

Between the casing member 1 and each of the positive electrode lead 11and the negative electrode lead 12, an adhesion film 2 is inserted forenhancing the adhesion of each of the positive electrode lead 11 and thenegative electrode lead 12 to the inside of the casing member 1 and forpreventing penetration of outside air. The adhesion film 2 is formed ofa material which has a property for adhesion to (secure contact with)each of the positive electrode lead 11 and the negative electrode lead12. Where the positive electrode lead 11 and the negative electrode lead12 are composed of the above-mentioned metallic materials, the adhesionfilm 2 is preferably formed, for example, of a polyolefin resin such aspolyethylene, polypropylene, modified polyethylene, modifiedpolypropylene, etc.

FIG. 2 is a sectional view, taken along line II of FIG. 1, of the woundelectrode body 10 shown in FIG. 1. The wound electrode body 10 has astructure in which a positive electrode 13 and a negative electrode 14are stacked together, with a separator 15 and an electrolyte 16therebetween, and an outermost peripheral portion of which is protectedby a protective tape 17.

[Positive Electrode]

The positive electrode 13, for example, has a positive electrode currentcollector 13A, and positive electrode active material layers 13Bprovided respectively on both sides of the positive electrode currentcollector 13A. Positive electrode active material layer may be providedonly one side of the positive electrode current collector 13A. As thepositive electrode current collector 13A, for example, a metallic foilsuch as an aluminum foil can be used.

The positive electrode active material layer 13B includes, as positiveelectrode active material, one or more than two sorts of positivematerials capable of occlusion and release electrode reaction substance.The positive electrode active material layer 13B, further, includes aconduction adjuvant such as a carbon material, and a binder such aspolyvinylidene fluoride or polytetrafluoroethylene.

[Positive Electrode Active Material]

The positive electrode active material is, for example, composite oxideparticles which contain therein a metallic element M2 different from aprincipal transition metal M1 and which have a concentration gradient ofthe metallic element M2 from the center toward the surface of eachparticle. The concentration gradient is such that the concentration ofthe metallic element M2 increases as the particle surface is approached.The composite oxide particles are particles of a lithium-containingtransition metal composite oxide wherein at least one element X selectedfrom among sulfur (S), phosphorus (P) and fluorine (F) is present in anaggregated form at the surfaces of the composite oxide particles.Incidentally, the state of the lithium-transition metal composite oxidesurface can be confirmed by observation of the obtained powder underSEM/EDX (Scanning Electron Microscopy/Energy Dispersive X-rayspectrometer).

The metallic element M2 is not particularly limited. It is preferable,however, that the composite oxide particles are particles of alithium-containing transition metal composite oxide prepared by aprocess wherein the metallic element M2 is preliminarily caused to bepresent inside the composite oxide particles, and the metallic elementM2 is allowed to react with a compound containing at least one element xselected from among sulfur (S), phosphorus (P) and fluorine (F) to raisethe concentration of the metallic element M2 at the particle surfaces.

Thus, the metallic element M2 is preliminarily distributed uniformlyinside the composite oxide particles and then the concentration of themetallic element M2 at the particle surfaces is raised, whereby themetallic element M2 can be made to be present uniformly at the particlesurfaces. Consequently, a modifying effect of the metallic element M2 onthe particle surfaces can be displayed to the utmost.

The metallic element M2 is preferably at least one element that canreplace, on a solid solution basis, the principal transition metalelement M1 in the inside of the composite oxide particles. Morepreferably, the metallic element M2 is at least one element selectedfrom the group consisting of manganese (Mn), magnesium (Mg), aluminum(Al), nickel (Ni), boron (B), titanium (Ti), cobalt (Co) and iron (Fe).It is effective for the metallic element M2 to be present at theparticle surfaces in the state of having replaced the principaltransition metal element A or in the state of having diffused into theinside near the surfaces of the particles to show a continuousconcentration gradient toward the center of each particle.

Incidentally, the concentration of magnesium can be confirmed by cuttinga section of the lithium-transition metal composite oxide and measuringthe element distributions in the radial direction by Auger electronspectroscopy.

Further, the reaction of the metallic element M2 with the compoundcontaining at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) conducted for raising the concentrationof the metallic element M at the particle surfaces is preferably carriedout in the co-existence of a lithium (Li) compound. Where the reactionis carried out in the co-existence of the Li compound, it is possible toregulate the amount of Li in the lithium-containing composite oxide andto suppress the lowering in capacity due to the surface modification.

As the lithium-transition metal composite oxide in the inside of theparticles, one of various known materials can be used. Preferably,however, the lithium-transition metal composite oxide is a materialwhich has a laminar rock salt structure and the principal transitionmetal element A as a constituent of which is at least selected fromamong nickel (Ni), cobalt (Co), manganese (Mn) and iron (Fe). Such amaterial promises a high capacity. Besides, known materials in whichsmall amounts of additive elements have been introduced as substituents,on a solid solution basis, can also be used.

Incidentally, the composite oxide particles serving as a base materialfor the positive electrode are, for example, lithium composite oxideparticles having a laminar rock salt structure and an averagecomposition represented by the following chemical formula (Chemical 1).The lithium composite oxide particles may be primary particles or may besecondary particles.

Li_(a)A_(b)M_(1-b)O_(c)  (Chemical 1)

In the formula, M is preferably at least one element selected from amongmanganese (Mn), magnesium (Mg), aluminum (Al), nickel (Ni), boron (B),titanium (Ti), cobalt (Co) and iron (Fe); a, b and c are numbers in theranges of 0.2≦a≦1.4, 0≦b≦1.0, and 1.8≦c≦2.2; and, incidentally, thecompositional ratio of lithium varies depending on thecharged/discharged state, the value of a shown here representing thevalue in a fully discharged state.

In the chemical formula (Chemical 1), the range of the value of a is,for example, 0.2≦a≦1.4. If the value of a is too small, the laminar rocksalt structure as the basic crystal structure of the lithium compositeoxide would be collapsed, whereby recharging would be made difficult toachieve and the capacity would be lowered considerably. If the value ofa is too large, on the other hand, lithium would diffuse to the outsideof the composite oxide particles, obstructing the control of basicity inthe subsequent process step and, finally, causing a trouble as topromotion of gelation during kneading of a positive electrode paste.

Incidentally, the lithium composite oxide in the above formula(Chemical 1) is so set that lithium may be contained in excess of itsamount according to the related art. Specifically, the value of a, whichrepresents the proportion of lithium in the lithium composite oxide ofthe above formula (Chemical 1), may be greater than 1.2. Here, the valueof 1.2 has been disclosed as the compositional ratio of lithium in thistype of lithium composite oxide in the related art, and, due to the samecrystal structure as in the case of a=1, gives a working-effectequivalent to that in the present application (refer to, for example,the present applicant's previous application: Japanese Patent Laid-openNo. 2008-251434).

Even when the value of a representing the compositional ratio of lithiumin the lithium composite oxide of the above formula (Chemical 1) isgreater than 1.2, the crystal structure of the lithium composite oxideis the same as in the case where the value of a is not more than 1.2.Besides, even when the value of a representing the compositional ratioof lithium in the above formula (Chemical 1) is greater than 1.2, if thevalue is not more than 1.4, the chemical states of the transition metalconstituting the lithium composite oxide in oxidation-reductionreactions attendant on the charge-discharge cycle are not much differentfrom those in the case where the value of a is not more than 1.2.

The range of the value of b is, for example, 0≦b≦1.0. If the value of bis reduced to below this range, the discharge capacity of the positiveelectrode active material would be lowered. If the value of b isincreased to above this range, on the other hand, stability of thecrystal structure of the composite oxide particles would be lowered,causing a lowering in the charge-discharge repetition capacity of thepositive electrode active material and a lowering in safety.

The range of value of c is, for example, 1.8≦c≦2.2. In the case wherethe value of c is below this range and in the case where the value isabove this range, stability of the crystal structure of the compositeoxide particles would be lowered, causing a lowering in thecharge-discharge repetition capacity of the positive electrode activematerial and a lowering in safety, and resulting in a reduction in thedischarge capacity of the positive electrode active material.

[Particle Diameter]

The positive electrode active material preferably has an averageparticle diameter of 2.0 to 50 μm. If the average particle diameter isless than 2.0 μm, exfoliation of a positive electrode active materiallayer would occur at the time of pressing the positive electrode activematerial layer during production of the positive electrode. In addition,due to an increased surface area of the positive electrode activematerial, it is necessary to increase the amounts of the conductivityadjuvant and the binder added, so that the energy density per unitweight tends to be lowered. If the average particle diameter exceeds 50μm, on the other hand, the particles tend to penetrate the separator,causing short-circuit.

The positive electrode 13 as above preferably has a thickness of notmore than 250 μm.

[Negative Electrode]

The negative electrode 14, for example, has a negative electrode currentcollector 14A, and negative electrode active material layers 14Bprovided respectively on both sides of the negative electrode currentcollector 14A. Negative electrode active material layer 14 b may beprovided only one side of the negative electrode current collector 13A.The negative electrode current collector 14A is composed, for example, ametallic foil such as a copper foil.

The negative electrode active material layer 14B is configured, forexample, to contain as a negative electrode active material at least onenegative electrode material capable of occlusion and release of lithium,and may contain a conductivity adjuvant and/or a binder, as required.

Examples of the negative electrode material capable of occlusion andrelease of lithium include carbon materials such as graphite, hardlygraphitizable carbon or easily graphitizable carbon, which may be usedeither singly or in mixture of two or more of them. Besides, two or moresuch materials differing in average particle diameter may be used inmixture.

Other examples of the negative electrode material capable of occlusionand release of lithium include those materials which contain as aconstituent element a metallic or semimetallic element capable offorming an alloy with lithium. Specific examples of such materialinclude elementary substances, alloys and compounds of metallic elementscapable of forming an alloy with lithium, as well as elementarysubstance, alloys and compounds of semimetallic elements capable offorming an alloy with lithium, and materials having a phase of one ormore of these elementary substances, alloys and compounds at least atpart thereof.

Examples of such metallic or semimetallic elements include tin (Sn),lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn), antimony(Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium(Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium(Y) and hafnium (Hf), among which preferred are the metallic orsemimetallic elements of Group 14 in the long-period periodic table, andparticularly preferred are silicon (Si) and tin (Sn). Silicon (Si) andtin (Sn) have a high capability of occlusion and release of lithium and,hence, promise a high energy density.

Examples of alloys of silicon (Si) include those alloys which contain atleast one selected from the group consisting of tin (Sn), nickel (Ni),copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium(In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony(Ab) and chromium (Cr), as a second constituent element other thansilicon (Si). Examples of alloys of tin (Sn) include those alloyscontaining at least one selected from the group consisting of silicon(Si), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn),zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge),bismuth (Bi), antimony (Ab) and chromium (Cr), as a second constituentelement other than tin (Sn).

Examples of the compounds of silicon (Si) or tin (Sn) include thosecompounds which contain oxygen (O) or carbon (C), and the compounds maycontain one or more of the above-mentioned second constituent elementsin addition to silicon (Si) or tin (Sn).

[Separator]

The separator 15 may be formed by use of any material that iselectrically stable, that is chemically stable in relation to thepositive electrode active material, the negative electrode activematerial and solvent, and that is not electrically conductive. Examplesof the material which can be used here include polymer nonwoven fabrics,porous films, and paper-like sheets of glass or ceramic fibers, whichmay be used in the form of a multi-layer laminate. Particularlypreferred are porous polyolefin films, which may be used in the form ofa composite with a heat-resistant material formed from polyimide, glassor ceramic fibers or the like.

[Electrolyte]

The electrolyte 16 includes an electrolytic solution and a retaineroperable to retain the electrolytic solution, the retainer including apolymeric compound, and is in a so-called gelled state. The electrolyticsolution contains an electrolytic salt and a solvent operable todissolve the electrolytic salt. Examples of the electrolytic saltinclude lithium salts such as LiPF₆, LiClO₄, LiBF₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, and LiAsF₆, which may be used either singly or in mixtureof two or more of them.

Examples of the solvent include nonaqueous solvents such as lactonesolvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone,ε-caprolactone, etc., carbonic acid ester solvents such as ethylenecarbonate, propylene carbonate, butylene carbonate, vinylene carbonate,dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, etc.,ether solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane,1,2-diethoxyethane, tetrehydrofuran, 2-methyltetrahydrofuran, etc.,nitrile solvents such as acetonitrile, etc., nonaqueous solvents such assulfolane solvents, phosphoric acids, phosphoric acid ester solvents,and pyrrolidones. These solvents may be used either singly or in mixtureof two or more of them.

Besides, the solvent preferably contains a compound having a structurein which hydrogen atoms of a cyclic ester or linear ester are partly orentirely fluorinated (replaced by fluorine atoms). Preferred as thefluorinated compound is difluoroethylene carbonate(4,5-difluoro-1,3-dioxolane-2-one). As a result of this,charge-discharge cycle characteristics can be enhanced even in the casewhere the negative electrode 14 including a compound of silicon (Si),tin (Sn), germanium (Ge) or the like as a negative electrode activematerial is used. Particularly, difluoroethylene carbonate has anexcellent improving effect on the cycle characteristics.

The polymeric compound may be any high molecular compound that is gelledthrough absorption of the solvent. Examples of the polymeric compoundinclude fluoro polymeric compounds such as polyvinylidene fluoride, avinylidene fluoride-hexafluoropropylene copolymer, etc., ether polymericcompounds such as polyethylene oxide, a polyethylene oxide-containingcrosslinked polymer, etc., and polymeric compounds containingpolyacrylonitrile, polypropylene oxide or polymethyl methacrylate as arepeating unit. These polymeric compounds may be used either singly orin mixture of two or more of them.

Particularly, from the viewpoint of oxidation-reduction stability, thefluoro polymeric compounds are desirable, among which the copolymerscontaining vinylidene fluoride and hexafluoropropylene as components arepreferable. Further, the copolymers may contain a monoester of anunsaturated dibasic acid such as monomethyl maleate, etc., an ethylenehalide such as ethylene trifluoride, etc., a cyclic carbonic acid esterof an unsaturated compound such as vinylene carbonate, or an epoxygroup-containing acrylvinyl monomer or the like as a component, wherebyit is made possible to obtain higher characteristics.

Furthermore, as a solid electrolyte, both inorganic solid electrolytesand polymeric solid electrolytes can be used insofar as the solidelectrolytes have lithium-ionic conductivity. Examples of the inorganicsolid electrolytes include lithium nitride and lithium iodide. Thepolymeric solid electrolytes each include an electrolytic salt and apolymeric compound operable to dissolve the electrolytic salt. Examplesof the polymeric compound include ether polymers such as poly(ethyleneoxide), its crosslinked product, etc., poly(methacrylate)ester polymers,acrylate polymers, and so on, which may be used singly, or as copolymersof two or more of them, or in mixture of two or more of them.

[Method of Producing Positive Electrode]

First, the composite oxide particles containing the metallic element M1in the present invention are synthesized. The means for synthesis of thecomposite oxide particles is not particularly limited. Further, as amethod for allowing the composite oxide particles to react with acompound containing at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) so as to raise the concentration of themetallic element M2 at the particle surfaces, various methods which havebeen known are applicable.

In addition, examples of the method for coating the surfaces of thecomposite oxide particles include a method in which a lithium-transitionmetal composite oxide containing the metallic element M2 and thecompound containing at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) are put into comminution, mixing andcoating (deposition) by use of a ball mill, a chaser mill, a pulverizeror the like. In carrying out this operation, it is effective to add anamount of a liquid component, which can be exemplified by water.Besides, coating (deposition) by a mechano-chemical treatment or coatingwith (deposition of) a metallic compound by a vapor phase method such assputtering, CVD (Chemical Vapor Deposition), etc. may also be adopted.

Further, surfaces containing at least one element selected from amongsulfur (S), phosphorus (P) and fluorine (F) can be formed on thelithium-transition metal composite oxide particles by mixing the rawmaterials in water or in such a solvent as ethanol, by crystallizationthrough neutralization in a liquid phase, or by other similar method.After at least one element selected from among sulfur (S), phosphorus(P) and fluorine (F) is thus made to be present on thelithium-transition metal composite oxide containing the metallic elementM2, a heat treatment is preferably carried out so as to raise theconcentration of the metallic element M2 at the particle surfaces. Theheat treatment may be carried out, for example, at a temperature of 350to 900° C. The lithium-transition metal composite oxide obtained may beone that has been treated by a known technology for control of powderproperties.

Subsequently, the positive electrode active material, the binder, andthe conduction adjuvant such as a carbon material are mixed together toprepare a positive electrode composition. The positive electrodecomposition is dispersed in a solvent such as N-methyl-2-pyrrolidone, toprepare a positive electrode composition slurry. The binder may bepolyvinylidene fluoride, polytetrafluoroethylene or the like.

Next, the positive electrode composition slurry is applied to thepositive electrode current collector 13A, and is dried. Thereafter,compression molding is carried out using a roll pressing machine or thelike so as to form the positive electrode active material layer 13B,thereby obtaining the positive electrode 13. Incidentally, theconductivity adjuvant such as a carbon material is mixed, as required,at the time of preparing the positive electrode composition.

Method of Producing Negative Electrode

Next, the negative electrode 14 is produced in the following manner.First, the negative electrode active material and the binder are mixedwith each other to prepare a negative electrode composition, and thenegative electrode composition is dispersed in a solvent such asN-methyl-2-pyrrolidone to prepare a negative electrode compositionslurry. Subsequently, the negative electrode composition slurry isapplied to the negative electrode current collector 14A, and the solventis evaporated off. Thereafter, compression molding is carried out usinga roll pressing machine or the like so as to form the negative electrodeactive material layer 14B, thereby obtaining the negative electrode 14.

Method of Manufacturing Nonaqueous Electrolyte Cell

The nonaqueous electrolyte cell can be manufactured, for example, in thefollowing manner. First, a precursor solution containing theelectrolytic solution, the polymeric compound and a mixed solvent isapplied to each of the positive electrode 13 and the negative electrode14, and the mixed solvent is evaporated off, to form the electrode 16.Thereafter, the positive electrode lead 11 is attached to an end portionof the positive electrode current collector 13A by welding, and thenegative electrode lead 12 is attached to an end portion of the negativeelectrode current collector 14A by welding.

Next, the positive electrode 13 and the negative electrode 14 with theelectrolyte 16 formed thereon are stacked together with the separator 15therebetween to form a stacked body, the stacked body is wound in thelongitudinal direction, and the protective tape 17 is adhered to anoutermost peripheral portion of the wound body, to form a woundelectrode body 10. Finally, for example, the wound electrode body 10 issandwiched between the casing members 1, and outer edge portions of thecasing members 1 are adhered to each other by heat fusing or the like,to seal the wounded electrode body 10 inside the casing members 1. Inthis case, the adhesion film 2 is inserted between each of the positiveelectrode lead 11 and the negative electrode lead 12 and each of thecasing members 1. In this way, the nonaqueous electrolyte cell as shownin FIGS. 1 and 2 is completed.

Besides, the nonaqueous electrolyte cell may also be manufactured in thefollowing manner. First, the positive electrode 13 and the negativeelectrode 14 are produced in the above-mentioned manner, and thepositive electrode lead 11 and the negative electrode lead 12 areattached to the positive electrode 13 and the negative electrode 14,respectively. Then, the positive electrode 13 and the negative electrode14 are stacked together with the separator 15 therebetween to form astacked body, the stacked body is wound, and the protective tape 17 isadhered to an outermost peripheral portion of the wound body, to form awound body as a precursor of the wound electrode body 10. Next, thewound body is sandwiched between the casing members 1, outer peripheraledge portions exclusive of one lateral edge of the casing members 1 areheat fused to obtain a bag-like shape, whereby the wound body isaccommodated inside the casing member 1. Subsequently, an electrolytecomposition containing the electrolytic solution, a monomer or monomersas raw material for the polymeric compound, a polymerization initiatorand, if necessary, other material(s) such as a polymerization inhibitoris prepared, and the electrolyte composition is introduced into theinside of the casing member 1.

After the introduction of the electrolyte composition, the openingportion of the casing member 1 is sealed by heat fusing in a vacuumatmosphere. Next, heat is applied to bring the monomer or monomers intopolymerization to form the polymeric compound, whereby the gelledelectrolyte 16 is formed and the nonaqueous electrolyte cell as shown inFIGS. 1 and 2 is assembled.

The details of the improvement of cycle characteristics and the likehave not yet been elucidated, but the improvement is supposed to beattained by the following mechanism. Inside the lithium ion secondarycell in the charged state, the positive electrode is in a stronglyoxidizing state, and the electrolytic solution in contact with thepositive electrode is in such an environment that oxidative dissociationthereof is liable to proceed, particularly in high-temperatureenvironments. When the dissociation of the electrolytic solutionproceeds, an inactive coating film is formed on the particles of thepositive electrode active material, thereby impeding the migration ofelectrons and/or lithium ions.

Further, the dissociated components produce very active molecules in theelectrolytic solution present in pores in the electrode, to acceleratethe deterioration of the electrolytic solution or to attack the positiveelectrode active material, thereby dissolving the component elements ofthe material or reducing the capacity. In order to restrain such aphenomenon, stabilization of the interface between the positiveelectrode active material particles and the electrolytic solution isinsufficient, and both this stabilization and stabilization of activemolecules outside of and in the vicinity of the positive electrodeactive material particles have to be achieved in a cooperative manner.

In the lithium-containing transition metal oxide in the embodiments, themetallic element M2 different from the principal transition metal insidethe oxide particles are made present at the particle surfaces, so as tostabilize the interface between the positive electrode active materialand the electrolytic solution. In addition, the lithium-transition metalcomposite oxide containing at least one element selected from amongsulfur (S), phosphorus (P) and fluorine (F) in an aggregated form ismade present in the vicinity of the particles, so as to stabilize theactive molecules. It is considered that, owing to a synergistic effectof these stabilizations, the cell performance is enhanced in a very highextent.

Further, it is considered that since the metallic element M2 ispreliminarily made to be uniformly present inside the particles and thenthe concentration of the metallic element M2 at the particle surfaces israised to ensure that the metallic element M is uniformly present at theparticle surfaces, the stabilizing effect of the metallic element M canbe displayed to the utmost, resulting in the successful enhancement ofcell performance.

Effect

According to the nonaqueous electrolyte cell in the first embodiment, itis possible to suppress deterioration of cycle characteristics, tosuppress a rise in internal resistance due to charge-discharge cycles inhigh-temperature environments, and thereby to simultaneously realizeboth an enhanced capacity and improved cell characteristics.

2. Second Embodiment Second Example of Nonaqueous Electrolyte Cell

A second embodiment will be described. A nonaqueous electrolyte cellaccording to the second embodiment uses positive active material withmore uniformly coating.

Since other materials and configurations are the same as in the firstembodiment, the explanations about these are omitted.

Positive Electrode Active Material

The positive electrode active material is, for example, composite oxideparticles which contain therein a metallic element M2 different from aprincipal transition metal M1 and which have a concentration gradient ofthe metallic element M2 from the center toward the surface of eachparticle. The concentration gradient is such that the concentration ofthe metallic element M2 increases as the particle surface is approached.The composite oxide particles are particles of a lithium-containingtransition metal composite oxide wherein at least one element X selectedfrom among sulfur (S), phosphorus (P) and fluorine (F) is present in anaggregated form at the surfaces of the composite oxide particles.

In the second embodiment, the compound containing at least one elementselected from among sulfur (S), phosphorus (P) and fluorine (F) or thepyrolyzed product of the compound, has a melting point of 70° C. or moreand of 600° C. or less. The compound or the pyrolyzed product of thecompound, being positioned on the surface of the composite oxide bycertain means such as using a ball mill, is melted by heating, touniformly coat the surface of the composite oxide. Following these, theheated and melted compound or pyrolyzed product of the compound is madeto react with the composite oxide. The coatings are more effectively anduniformly made than those of the first embodiment.

When the compound or pyrolyzed product of the compound is heated at thetemperature of more than 600° C., the reaction in the composite oxidewill be brought about and the structure of the composite oxide can bechanged. But in this embodiment, the compound or the pyrolyzed productof the compound melts and coats the composite oxide to react with thecomposite oxide to be in stable status, before the change of thestructure of the composite oxide.

If the melting point of the compound or pyrolyzed product is more than600° C., the coating reaction begins before the compound or the productmelts and coat the surface of the composite oxide and reaction betweenthe compound or the product and the composite oxide starts, whichrenders partial coating reactions only at the site where the compound orthe product and the composite oxide are in contact, which leads to theunfavorable nonuniform coating on the composite oxide.

Heating at the temperature of more than 600° C. also leads to a changein the structure of the composite oxide.

If the melting point of the compound or pyrolyzed product is less than70° C., the component or the product will unfavorably melt or decomposeduring deposition by ball milling or the like.

The compound or the pyrolyzed product of the compound has preferably theaverage diameter of 30 μm or less. The compound or the pyrolyzed productof such diameter will realize the uniform coatings of the compositeoxide. When the diameter of the compound or the pyrolyzed product is toolarge, they cannot be well mixed with the composite oxide, which bringsabout the nonuniform deposition on the composite oxide with a ball millor the like. The diameter of the compound or the pyrolyzed product hasno lower limitat. A smaller diameter will enable more uniform coatings.But the diameter is practically limited by the communition of thecompound or the pyrolyzed product into about 1 μm.

Examples of the compound are ammonium phosphate dibasic ((NH₄)₂HPO₄),ammonium dihydrogen phosphate (NH₄H₂PO₄), ammonium sulfate ((NH₄)₂HPO₄),phosphoric acid (H₃PO₄), and so on. Cations of these are eliminated byfor example, vaporization when heated and, therefore, the positiveelectrode active materials without impurities can be obtained, which canavoid decrease of the capacity and other adverse effects.

As a metallic element M2 different from the principal transition metalM1, the same metallic element M2 as in the embodiment 1 can be adopted.

Method of Producing Positive Electrode Active Material

Positive electrode active material of the second embodiment can beprepared, for example, according to the following procedures.

First, the surfaces of the composite oxide particles are coated withcoating materials. As for the example method for coating the surfaces ofthe composite oxide particles, a method the same as in the firstembodiment can be adopted, in which a lithium-transition metal compositeoxide containing the metallic element M1 and the compound containing atleast one element X selected from among sulfur (S), phosphorus (P) andfluorine (F) are put into comminution, mixing and coating (deposition)by use of a ball mill, a chaser mill, a pulverizer or the like.

In carrying out this operation, it is effective to add an amount of aliquid component, which can be exemplified by water. Besides, coating(deposition) by a mechano-chemical treatment or coating with (depositionof) a metallic compound by a vapor phase method such as sputtering, CVD(Chemical Vapor Deposition), etc. may also be adopted.

After at least one element X selected from among sulfur (S), phosphorus(P) and fluorine (F) is thus made to be present on thelithium-transition metal composite oxide containing the metallic elementM1, a heat treatment is preferably carried out so as to raise theconcentration of the metallic element M2 at the particle surfaces. Theheat treatment may be carried out, for example, at a temperature of 700to 900° C. The lithium-transition metal composite oxide obtained may beput through the treatment of a known technology for control of powderproperties or some other purposes.

During the heat treatment, the compound positioned on the surface of thecomposite oxide is melted to be in a liquid state and the surface of thecomposite oxide gets uniformly covered with the compound. After afurther heat treatment, the compound is decomposed and cations areeliminated, and anions react with metallic element M2 included incomposite oxide. The temperature of the heat treatment can be raisedafter the compound is melted to react the compound with coatingmaterial.

Effect

According to the second embodiment, the composite oxide can be coatedwith coating materials before the structure of the composite oxidechanges. Therefore the function of the cathode active material isimproved, which leads to better properties of nonaqueous electrolytesecondary battery.

3. Third Embodiment Third Example of Nonaqueous Electrolyte Cell

A second embodiment will be described. A nonaqueous electrolyte cellaccording to the second embodiment uses an electrolytic solution inplace of the gelled electrolyte 16 in the nonaqueous electrolyte cellaccording to the first embodiment. In this case, the electrolyticsolution is used by impregnating the separator 15 therewith. As theelectrolytic solution, the same electrolytic solutions as those in thefirst embodiment above can be used.

The nonaqueous electrolyte cell thus configured can be manufactured, forexample, in the following manner. First, the positive electrode 13 andthe negative electrode 14 are produced. The positive electrode 13 andthe negative electrode 14 can be produced in the same manner as in theabove-described first embodiment, and, therefore, detailed descriptionof the production is omitted here.

Next, after the positive electrode lead 11 and the negative electrodelead 12 are attached respectively to the positive electrode 13 and thenegative electrode 14, the positive electrode 13 and the negativeelectrode 14 are stacked together with the separator 15 therebetween toform a stacked body, the laminate is wound, and the protective tape 17is adhered to an outermost peripheral portion of the wound body.

As a result, a wound electrode body is obtained which is the same inconfiguration as the above-described wound electrode body 10, exceptthat the electrolyte 16 is omitted. After the wound body is sandwichedbetween the casing members 1, the electrolytic solution is introducedinto the inside of the casing members 1, and the casing members 1 aresealed off. In this manner, the nonaqueous electrolyte cell according tothe second embodiment is obtained.

Effect

According to the third embodiment, the effects equivalent to those inthe first embodiment above can be obtained. Specifically, it is possibleto suppress deterioration of cycle characteristics, to suppress a risein internal resistance due to charge-discharge cycles inhigh-temperature environments, and thereby to simultaneously realizeboth an enhanced capacity and improved cell characteristics.

4. Fourth Embodiment Fourth Example of Nonaqueous Electrolyte Cell

Now, the configuration of a nonaqueous electrolyte cell according to afourth embodiment will be described below referring to FIGS. 3 and 4.FIG. 3 shows the configuration of the nonaqueous electrolyte cellaccording to the fourth embodiment.

This nonaqueous electrolyte cell is a so-called cylindrical type cell,wherein a wound electrode body 30 formed by winding a band-shapedpositive electrode 31 and a band-shaped negative electrode 32 with aseparator 33 therebetween is disposed inside a substantially hollowcylindrical cell can 21.

The separator 33 is impregnated with an electrolytic solution, which isa liquid electrolyte. The cell can 21 is formed, for example, from iron(Fe) plated with nickel (Ni). The cell can 21 is closed at one endthereof, and is open at the other end thereof. Inside the cell can 21, apair of insulating plates 22 and 23 are disposed perpendicularly to thecircumferential surface of the wound electrode body 30, respectively onopposite sides of the wound electrode body 30.

At the open end of the cell can 21, a cell cap 24 as well as a safetyvalve mechanism 25 and a PTC (Positive Temperature Coefficient)thermistor element 26 which are provided inside the cell cap 24 ismounted by caulking, through using a gasket 27. By this, the inside ofthe cell can 21 is sealed off.

The cell cap 24 is formed, for example, from the same material as thatof the cell can 21. The safety vale mechanism 25 is electricallyconnected with the cell cap 24 through the thermistor element 26. Thesafety valve mechanism 25 is so configured that when the internalpressure of the cell reaches or exceeds a predetermined value due tointernal short-circuit or external heating, a disc plate 25A is invertedto cut off the electrical connection between the cell cap 24 and thewound electrode body 30.

The thermistor element 26 limits a current by an increase in itsresistance when temperature is raised, so as to prevent abnormal heatgeneration from occurring due to a large current. The gasket 27 isformed, for example, from an insulating material, and its surface iscoated with asphalt.

The wound electrode body 30 is, for example, wound about a center pin34. A positive electrode lead 35 formed from aluminum (Al) or the likeis connected to the positive electrode 31 in the wound electrode body30, while a negative electrode lead 36 formed from nickel (Ni) or thelike is connected to the negative electrode 32. The positive electrodelead 35 is electrically connected to the cell cap 24 by being welded tothe safety valve mechanism 25, while the negative electrode lead 36 iselectrically connected to the cell can 21 by welding to the latter.

FIG. 4 is a sectional view showing, in an enlarged form, a part of thewound electrode body 30 shown in FIG. 3. The wound electrode body 30 hasa structure in which the positive electrode 31 and the negativeelectrode 32 are stacked together with the separator 33 therebetween toform a stacked body, and the stacked body is wound.

The positive electrode 31, for example, includes a positive electrodecurrent collector 31A and positive electrode active material layers 31Bprovided respectively on both sides of the positive electrode currentcollector 31A. The negative electrode 32, for example, includes anegative electrode current collector 32A and negative electrode activematerial layers 32B provided respectively on both sides of the negativeelectrode current collector 32A. The configurations of the positiveelectrode current collector 31A, the positive electrode active materiallayers 31B, the negative electrode current collector 32A, the negativeelectrode active material layers 32B, the separator 33 and theelectrolytic solution are equivalent respectively to those of thepositive electrode current collector 13A, the positive electrode activematerial layers 13B, the negative electrode current collector 14A, thenegative electrode active material layers 14B, the separator 15 and theelectrolytic solution in the first embodiment described above.

Method of Manufacturing Nonaqueous Electrolyte Cell

Now, the method of manufacturing the nonaqueous electrolyte cellaccording to the fourth embodiment will be described below. The positiveelectrode 31 is produced as follows. First, a positive electrode activematerial and a binder are mixed with each other to prepare a positiveelectrode composition, which is dispersed in a solvent such asN-methyl-2-pyrrolidone to prepare a positive electrode compositionslurry. Next, the positive electrode composition slurry is applied tothe positive electrode current collector 31A, and is dried. Thereafter,compression molding is carried out using a roll pressing machine or thelike to form the positive electrode active material layers 31B, therebyobtaining the positive electrode 31.

The negative electrode 32 is produced in the following manner. First, anegative electrode active material and a binder are mixed with eachother to prepare a negative electrode composition, which is dispersed ina solvent such as N-methyl-2-pyrrolidone to prepare a negative electrodecomposition slurry. Next, the negative electrode composition slurry isapplied to the negative electrode current collector 32A, and the solventis evaporated off. Thereafter, compression molding is conducted using aroll pressing machine or the like to form the negative electrode activematerial layers 32B, thereby obtaining the negative electrode 32.

Subsequently, a positive electrode lead 35 is attached to the positiveelectrode current collector 31A by welding or the like, and a negativeelectrode lead 36 is attached to the negative electrode currentcollector 32A by welding or the like. Thereafter, a stack of thepositive electrode 31 and the negative electrode 32 with the separator33 therebetween is wound, a tip portion of the positive electrode lead35 is welded to the safety valve mechanism 25, and a tip portion of thenegative electrode lead 36 is welded to the cell can 21.

Then, the stack of the positive electrode 31 and the negative electrode32 is clamped between the pair of insulating plates 22 and 23, and areaccommodated in the cell can 21. After the positive electrode 31 and thenegative electrode 32 are accommodated in the cell can 21, theelectrolyte is introduced into the inside of the cell can 21 so that theseparator 33 is impregnated with the electrolyte.

Thereafter, the cell cap 24, the safety valve mechanism 25 and thethermistor element 26 are fixed to the open end portion of the cell can21 by caulking, through using the gasket 27. In this manner, thenonaqueous electrolyte cell shown in FIG. 3 is manufactured.

Effect

In the nonaqueous electrolyte cell according to the fourth embodiment,it is possible to suppress generation of gas and to prevent breakage ofthe cell due to a rise in internal pressure.

5. Fifth Embodiment Fifth Example of Nonaqueous Electrolyte Cell

A nonaqueous electrolyte cell according to a fifth embodiment uses apositive electrode active material with more uniform coating in place ofthe positive electrode active material in the nonaqueous electrolytecell of the fourth embodiment.

Since other materials and configurations are the same as in the fourthembodiment, the explanations about these are omitted.

Positive Electrode Active Material

The molar faction r(%) Of the positive electrode active material of thefifth embodiment satisfies the formula 0.20≦r≦0.80 in the range wherethe ratio d(%) from the surface to a certain depth satisfies0.020≦d≦0.050. Ratio d and molar fraction r are determined according tothe following formula.

Ratio d(%)=[(mass of the principal transition metal M1)+(mass ofmetallic element M2)]/(mass of whole particles)  (I)

Molar fraction r=(mass of metallic element M2)/[(mass of principaltransition metal M1)+(mass of metallic element M2)]  (II)

Except the above-mentioned point, the positive electrode active materialof the fifth embodiment is same as that of the fourth embodiment.

The mass of a principal transition metal Ma and the mass of the metallicelement M2 can be known by dissolving the surface of thelithium-transition metal composite oxide into a buffering solvent toanalyze the mass of the content of a principal transition metal Ma andmetallic element M2 dissolved in the buffering solvent.

Specifically ratio d(%) and molar fraction rate r can be determined asfollows. First, add buffering solvent is added to lithium-transitionmetal composite oxide particle and they are mixed. Then, the bufferingsolvent is sampled every certain term and the solvent is filtered. Themass of the principal transition metal M1 and mass of the metallicelement M2 contained in each of the buffering solvent are measured byInductively Coupled Plasma method.

Then the amounts [mol] of metal M1 and metallic element M2 arecalculated from the masses, and obtain ratio d and molar fraction r areobtained according to the formula (I) and (II). Here, the particle ispresumed to be spherical and calculation is made under the presumptionthat the particles dissolved into the buffering solvent get smaller indiameter in a state of maintaining spherical form.

The above mentioned analysis of the surface of the positive electrodeactive material is three dimensional and enables quantitative analysisof the concentration gradient, which are quite difficult to realize bythe conventional analysis method of the surface status of positiveelectrode active material.

On condition that molar fraction ratio r(%) falls within the range of0.20≦r≦0.80 where the ratio d(%) from the surface to a certain depthsatisfies 0.020≦d≦0.050, capacity retention and high-temperaturepreservability capacity are high.

Whereas even if molar fraction ratio r(%) falls in the range of0.20≦r≦0.80 where the ratio d(%) from the surface to a certain depthdoes not satisfy 0.020≦d≦0.050, there is a tendency of the effect ofimprovement in capacity retention and high-temperature storagepreservability not always being shown.

It is preferable that the molar ratio r(%) decreases from the surface tothe inside within the range where the ratio from the surface to acertain depth d(%) satisfies 0.020≦d≦0.050, because lowering of capacityretention rate and high-temperature preservability are avoidable,especially, lowering of capacity retention is remarkably avoidable.

In addition to that the molar fraction ratio r(%) falls within the rangeof 0.20≦r≦0.80 where the ratio d(%) from the surface to a certain depthsatisfies 0.020≦d≦0.050, it is preferable that molar ratio r satisfies0.55≦r≦1.0 in the range where the ratio from the surface to a certaindepth d(%) satisfies 0.010≦d≦0.020, because lowering of dischargecapacity is avoided and cycle property and high-temperature storageproperty are improved.

Method of Producing Cell

Method of producing the nonaqueous electrolyte secondary cell of thefifth embodiment is as follows.

First, lithium-transition metal composite oxide particles containinglithium, a principal transition metal M1, and metallic element M2 aremixed with a compound containing at least one element X selected fromamong sulfur (S), phosphorus (P) and fluorine (F). A compound includinglithium is preferably further mixed. Then, deposition of the compoundcontaining at least one element X selected from among sulfur (S),phosphorus (P) and fluorine (F) and preferably the compound includinglithium on the surface of the lithium-transition metal composite oxideis effected by mechanochemical treatment. The mixture is under themechanochemical treatment for 5 minutes or longer and two hours orshorter. Coating is not enough when mechanochemical treatment is doneshorter than 5 minutes, while the positive electrode active materialparticles break into smaller particles with too short diameters.

Next, lithium-transition metal composite oxide particle is fired to getpositive electrode active material. The temperature for firing ispreferably 500 to 1500° C. If the temperature is under 500° C., thelithium-transition metal particles are not coated enough. While, if thetemperature is higher than 1500° C., particles aggregate to makesecondary particles, which leads to worsening of coating properties oncollectors.

After firing, lithium-transition metal composite oxide particles have aconcentration gradient of the metallic element M2 from the center towardthe surface of each particle. The particles contains at least oneelement X selected from among sulfur (S), phosphorus (P) and fluorine(F) deposited on surfaces of the composite oxide particles in the formof aggregate.

Generally molar ratio r can be adjusted by adapting addition amount ofthe compound containing at least one element X selected from amongsulfur (S), phosphorus (P) and fluorine (F). When the compound is addedtoo little, reaction is too small to obtain enough coating and molarratio r is lowered. When addition amount is large, molar ratio r getslarger, but r never becomes larger than 1 in principle. The reactionproceeds from the surface to the inside, and therefore when additionalamount is large, a high molar fraction ratio is obtained from the partswhere the ratio d(%) from the surface to a certain depth is large.

When coating materials, i.e., the compound or the pyrolysed compound andthe base materials i.e., the lithium-transition metal composite oxideare not well mixed, the molar fractional ratio r is lowered. Forexample, diameter of the compound is 100 μm or more, which is largerthan the average diameter of positive electrode material 5 to 30 μm, andnonuniformly dispersed. Consequently preferable coating status is notobtained and molar fraction ratio r sometimes gets low. As for techniqueof mixing, any techniques can be adapted so long as base materials andcoating materials are well mixed, such as planetary mixer, primarytechniques like shaking mixtures in the bag.

After the positive electrode active material is obtained, the sameprocedures as described in the fourth embodiment can be taken to obtainnonaqueous electrolyte cell of the fifth embodiment.

The upper limit of charging voltage of the cell of the fourth embodimentcan be 4.2 V, but preferably is designed to be higher than 4.2 V.Practically, cell is designed so that the upper limit of chargingvoltage is preferably 4.25 to 4.80 V, more preferably 4.35 V or higherfrom the viewpoint of discharge capacity, 4.65 V or lower from theviewpoint of safety. The lower limit of discharging voltage of the cellis preferably 2.00 to 3.30 V. To design cell voltage high brings about ahigh energy density.

6. Other Embodiment Modification

The present invention is not limited to the above-described embodimentsof the invention, and various modifications and applications arepossible within the scope of the invention. For instance, the shape ofthe nonaqueous electrolyte cell is not limited to the above-mentionedtype (cylindrical type), and may be of a coin type, for example.

In addition, for example, a polymeric solid electrolyte included anionically conductive polymeric material or an inorganic solidelectrolyte included an ionically conductive inorganic material may beused as the electrolyte. Examples of the ionically conductive polymericmaterial include polyethers, polyesters, polyphosphazenes, andpolysiloxanes. Examples of the inorganic solid electrolyte includeionically conductive ceramics, ionically conductive crystals, andionically conductive glasses.

The positive electrode active material of the fifth embodiment can beadopted in the cell of the first to third embodiment.

EXAMPLES

Now, the present invention will be described specifically below byshowing examples, which are not to be construed as limitative of theinvention.

In the examples 1-1 to 1-13 and comparative examples 1-1 to 1-9,additional volume of coating materials was varied and cell propertieswith positive materials different in distribution of coating materialsat the surface of composite oxide were determined.

Example 1-1 Production of Positive Electrode

After lithium carbonate (Li₂CO₃), cobalt oxide (Co₃O₄), aluminumhydroxide (Al(OH)₃), and magnesium carbonate (MgCO₃) were mixed at amolar ratio of Li:Co:Al:Mg=1.00:0.98:0.01:0.01, the mixture was fired at900° C. in air for 5 hr, to obtain a lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂). The average particle diameter of thelithium-cobalt composite oxide was measured by a laser scatteringmethod, to be 13 μm.

Subsequently, lithium carbonate (Li₂CO₃) and diammoniumhydrogenphosphate ((NH₄)₂HPO₄) were weight and mixed with thelithium-cobalt composite oxide (LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) so asto obtain an atomic ratio of Co:Li:P=98:1:1. Then, the mixed materialcontaining the lithium-cobalt composite oxide was treated by amechano-chemical system for 1 hr. As a result, there was obtained aprecursor before firing in which particles of the lithium-cobaltcomposite oxide were present as a central material, and lithiumcarbonate and diammonium hydrogenphosphate were deposited on theparticle surfaces.

The precursor before firing was subjected to a temperature rise at arate of 3° C./min, and was held at 900° C. for 3 hr, followed by slowcooling, to obtain a lithium-transition metal composite oxide pertainingto the present invention. The lithium-transition metal composite oxidehad magnesium (Mg) uniformly distributed at the surfaces of thelithium-cobalt composite oxide particles. Besides, the concentration ofmagnesium (Mg) was higher at the particle surfaces than in the inside ofthe particles, and lithium phosphate (Li₃PO₄) was interspersed on theparticle surfaces.

Incidentally, the surface state of the lithium-transition metalcomposite oxide was confirmed by observation of the obtained powderunder SEM/EDX. Upon the observation of the surfaces of thelithium-transition metal composite oxide, uniform distribution ofmagnesium (Mg) on the particle surfaces and interspersing of phosphoruson the particle surfaces were confirmed. In addition, magnesiumconcentration was confirmed by cutting a section of thelithium-transition metal composite oxide, and measuring the elementdistribution in the radial direction by Auger electron spectroscopy.Upon the measurement of the element distribution in the section of thelithium-transition metal composite oxide, the magnesium concentrationwas confirmed to be varying continuously from the surface toward theinside of the particle.

In addition, when the powder was subjected to measurement of powderX-ray diffraction pattern by use of CuKα, a diffraction peakcorresponding to Li₃PO₄ was confirmed in addition to a diffraction peakcorresponding to LiCoO₂ having a laminar rock salt structure.

By using as a positive electrode active material the lithium-transitionmetal composite oxide obtained as above, a nonaqueous electrolytesecondary cell was manufactured, and the cell was evaluated with respectto high-temperature cycle characteristics and internal resistancevariation, as described below.

A positive electrode composition was prepared by mixing 98 wt % of theabove-mentioned positive electrode active material, 1.5 wt % of anamorphous carbon powder (Ketchen black) and 3 wt % of polyvinylidenefluoride (PVdF). The positive electrode composition was dispersed inN-methyl-2-pyrrolidone (NMP) to prepare a positive electrode compositionslurry, which was then uniformly applied to both sides of a positiveelectrode current collector composed of a band-shaped aluminum foil.Subsequently, the positive electrode composition slurry on the surfacesof the positive electrode current collector was dried in a stream ofwarm air, and compression molding was carried out using a roll pressingmachine, to form positive electrode composition layer.

Production of Negative Electrode

A negative electrode composition was prepared by mixing 95 wt % of agraphite powder with 5 wt % of PVdF. The negative electrode compositionwas dispersed in N-methyl-2-pyrrolidone to prepare a negative electrodecomposition slurry, which was then uniformly applied to both sides of anegative electrode current collector composed of a band-like copperfoil, followed by press molding under heating, to form negativeelectrode composition layers.

Preparation of Electrolytic Solution

In a mixed solvent obtained by mixing ethylene carbonate (EC) and methylethyl carbonate (MEC) in a volume ratio of 1:1, lithiumhexafluorophosphate (LiPF₆) as dissolved so as to attain a concentrationof 1 mol/dm³, to prepare a nonaqueous electrolytic solution.

Assembly of Cell

The band-shaped positive and negative electrodes produced as above werewound a number of times in the state of being stacked together, with aseparator composed of a porous polyolefin therebetween, to produce aspiral type wound electrode body. The wound electrode body wasaccommodated in a cell can made of nickel-plated iron, and insulatingplates were disposed on upper and lower faces of the wound electrodebody. Next, a nickel-made negative electrode terminal connected with anegative electrode current collector was welded to a bottom portion ofthe cell can. In addition, an aluminum-made positive electrode terminalconnected with a positive electrode current collector was welded to aprotruding portion of a safety valve securely put in electricalconduction to a cell cap.

Finally, the nonaqueous electrolytic solution was introduced into thecell can in which the wound electrode body had been incorporated.Thereafter, the cell can was caulked through using an insulating sealgasket, to immobilize the safety valve, a PTC thermistor element and thecell cap. In this manner, there was manufactured a cylindrical type cellhaving an outside diameter of 18 mm and a height of 65 mm.

Evaluation of Cell

(a) Initial Capacity

The cylindrical type cell manufactured as above was subjected toconstant-current charging at a charging current of 1.5 A up to acharging voltage of 4.35 V in an environment of an ambient temperatureof 45° C. Then, the constant-current charging was switched over toconstant-voltage charging, and the charging was finished when the totalcharging time reached 2.5 hr. Immediately thereafter, the cell wassubjected to discharging at a discharging current of 2.0 A, and thedischarging was finished when the cell voltage was lowered to 3.0 V. Thedischarge capacity in this instance was measured as an initial capacity,which was found to be 9.1 Wh.

(b) Capacity Retention

The cell was subjected to repeated charge-discharge cycles in the samecharge-discharge conditions as in the above-mentioned case for measuringthe initial capacity. Upon the 300th cycle, the discharge capacity wasmeasured, and capacity retention based on the initial capacity wasdetermined. The capacity retention was 82%.

Example 1-2

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the cell voltage upon charging was4.20 V. Upon evaluation of the cell, the initial capacity was found tobe 8.0 Wh, and the capacity retention was 82%. Incidentally, theconcentration distributions of elements in the positive electrode activematerial and the surface states of particles of the active material inExample 1-2 and the latter Examples and Comparative Examples are shownin Table 1 below.

Example 1-3

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the cell voltage upon charging was4.4 V. Upon evaluation of the cell, the initial capacity was found to be9.4 Wh, and the capacity retention was 80%.

Example 1-4

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the cell voltage upon charging was4.5 V. Upon evaluation of the cell, the initial capacity was found to be10.0 Wh, and the capacity retention was 61%.

Example 1-5

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating material to bedeposited on lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) was ammonium dihydrogenphosphate(NH₄H₂PO₄). Upon evaluation of the cell, the initial capacity was foundto be 9.1 Wh, and the capacity retention was 80%.

Example 1-6

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating material to bedeposited on lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) was lithium hexafluorophosphate(LiPF₆) and the firing temperature was 700° C. Upon evaluation of thecell, the initial capacity was found to be 9.1 Wh, and the capacityretention was 81%.

Example 1-7

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating material to bedeposited on lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) was lithium tetrafluoroborate (LiBF₄)and the firing temperature was 700° C. Upon evaluation of the cell, theinitial capacity was found to be 9.1 Wh, and the capacity retention was76%.

Example 1-8

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating material to bedeposited on lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) was sulfur (S), and the firingtemperature of 700° C. Upon evaluation of the cell, the initial capacitywas found to be 9.1 Wh, and the capacity retention was 64%.

Example 1-9

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂), lithium carbonate (Li₂CO₃) anddiammonium hydrogenphosphate ((NH₄)₂HPO₄) were mixed in an atomic ratioof Co:Li:P=98:0.5:0.5. Upon evaluation of the cell, the initial capacitywas found to be 9.1 Wh, and the capacity retention was 80%.

Example 1-10

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂), lithium carbonate (Li₂CO₃) anddiammonium hydrogenphosphate ((NH₄)₂HPO₄) were mixed in an atomic ratioof Co:Li:P=98:2.5:2.5. Upon evaluation of the cell, the initial capacitywas found to be 8.9 Wh, and the capacity retention was 75%.

Example 1-11

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that lithium-cobalt composite oxide(LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂), lithium carbonate (Li₂CO₃) anddiammonium hydrogenphosphate ((NH₄)₂HPO₄) were mixed in an atomic ratioof Co:Li:P=98:5:5. Upon evaluation of the cell, the initial capacity wasfound to be 8.2 Wh, and the capacity retention was 69%.

Example 1-12

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCo_(0.97)Al_(0.01)Mg_(0.02)O₂. Upon evaluation ofthe cell, the initial capacity was found to be 9.0 Wh, and the capacityretention was 84%.

Example 1-13

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCo_(0.95)Al_(0.01)Mg_(0.04)O₂. Upon evaluation ofthe cell, the initial capacity was found to be 8.8 Wh, and the capacityretention was 82%.

Comparative Example 1-1

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating treatment oflithium-cobalt composite oxide (LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) wasomitted. Upon evaluation of the cell, the initial capacity was found tobe 9.2 Wh, and the capacity retention was 31%.

Comparative Example 1-2

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating treatment oflithium-cobalt composite oxide (LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) wasomitted and that the cell voltage upon charging was 4.2 V. Uponevaluation of the cell, the initial capacity was found to be 8.1 Wh, andthe capacity retention was 71%.

Comparative Example 1-3

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the coating treatment oflithium-cobalt composite oxide (LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂) wasomitted and that the cell voltage upon charging was 4.4 V. Uponevaluation of the cell, the initial capacity was found to be 9.5 Wh, andthe capacity retention was 25%.

Comparative Example 1-4

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCoO₂, that the coating material to be deposited onlithium-cobalt composite oxide (LiCoO₂) was a mixture of lithiumcarbonate (Li₂CO₃), magnesium carbonate (MgCO₃) and diammoniumhydrogenphosphate ((NH₄)₂HPO₄), and that lithium-cobalt composite oxide(LiCoO₂), lithium carbonate (Li₂CO₃), magnesium carbonate (MgCO₃) anddiammonium hydrogenphosphate ((NH₄)₂HPO₄) were so weighed and mixed asto obtain an atomic ratio of Co:Li:Mg:P=100:1:1:1. Upon evaluation ofthe cell, the initial capacity was found to be 9.1 Wh, and the capacityretention was 32%.

Comparative Example 1-5

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCoO₂, that the coating material to be deposited onlithium-cobalt composite oxide (LiCoO₂) was aluminum fluoride (AlF₃),and that lithium-cobalt composite oxide (LiCoO₂) and aluminum fluoride(AlF₃) were so weighed and mixed as to obtain an atomic ratio ofCo:Al=100:1. Upon evaluation of the cell, the initial capacity was foundto be 9.1 Wh, and the capacity retention was 30%.

Comparative Example 1-6

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCoO₂, that the coating material to be deposited onlithium-cobalt composite oxide (LiCoO₂) was aluminum phosphate (AlPO₄),and that lithium-cobalt composite oxide (LiCoO₂) and aluminum phosphate(AlPO₄) were so weighed and mixed as to obtain an atomic ratio ofCo:Al=100:1. Upon evaluation of the cell, the initial capacity was foundto be 9.1 Wh, and the capacity retention was 25%.

Comparative Example 1-7

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCoO₂. Upon evaluation of the cell, the initialcapacity was found to be 9.1 Wh, and the capacity retention was 20%.

Comparative Example 1-8

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the composition of lithium-cobaltcomposite oxide was LiCoO₂, that the coating material to be deposited onlithium-cobalt composite oxide (LiCoO₂) was lithium phosphate (Li₃PO₄),and that lithium-cobalt composite oxide (LiCoO₂) and lithium phosphate(Li₃PO₄) were so weighed and mixed as to obtain an atomic ratio ofCo:P=100:1. Upon evaluation of the cell, the initial capacity was foundto be 9.1 Wh, and the capacity retention was 15%.

Comparative Example 1-9

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Example 1-1, except that the firing temperature during thefiring after the treatment for coating with diammonium hydrogenphosphate((NH₄)₂HPO₄) was 300° C. Upon evaluation of the cell, the initialcapacity was found to be 8.6 Wh, and the capacity retention was 35%.

The evaluation results are shown in Table 1 below.

TABLE 1 Composite Distr. States Firing Initial Capacity oxide CoatingAddition Mg conc. temp. Voltage capacity retention particle materialamounts Mg gradient Surface [° C.] [V] [Wh] [%] Example 1-1Lic_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ Co:Li:P = 98:1:1 u. pr. P, int.900 4.35 9.1 82 Example 1-2 (NH₄)₂HPO Co:Li:P = 98:1:1 u. pr. P, int.900 4.2 8.0 85 Example 1-3 Co:Li:P = 98:1:1 u. pr. P, int. 900 4.4 9.480 Example 1-4 Co:Li:P = 98:1:1 u. pr. P, int. 900 4.5 10.0 61 Example1-5 (NH₄)₂HPO₄ Co:P = 98:1 u. pr. P, int. 900 4.35 9.1 80 Example 1-6LiPF₆ Co:Li:P = 98:1:1 u. pr. P, F, int. 700 4.35 9.1 81 Example 1-7LiBF₄ Co:Li:P = 98:1:1 u. pr. F, int. 700 4.35 9.1 76 Example 1-8 S Co:S= 98:1 u. pr. S, int. 700 4.35 9.1 64 Example 1-9 Li₂CO₃ Co:Li:P =98:0.5:0.5 u. pr. P, int. 900 4.35 9.1 80 Example 1- (NH₄)₂HPO₄ Co:Li:P= 98:0.5:0.5 u. pr. P, int. 900 4.35 8.9 75 10 Example 1- Co:Li:P =98:5:5 u. pr. P, int. 900 4.35 8.2 69 11 Example 1-LiCo_(0.96)Al_(0.01)Mg_(0.02)O₂ Co:Li:P = 97:1:1 u. pr. P, int. 900 4.359.0 84 12 Example 1- LiCo_(0.94)Al_(0.01)Mg_(0.04)O₂ Co:Li:P = 95:1:1 u.pr. P, int. 900 4.35 8.8 82 13 Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂— — u. ab. — 900 4.35 9.2 31 1-1 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ — — u. ab. — 900 4.2 8.1 71 1-2 Comp.Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ — — u. ab. — 900 4.4 9.5 25 1-3LiCoO₂ Li₂CO₃ Comp. Ex. MgCO₃ Co:Li:Mg:P = 100:1:1:1 nu. pr. P, int. 9004.35 9.1 32 1-4 (NH₄)₂HPO₄ Comp. Ex. LiCoO₂ AlF₃ Co:A1 = 100:1 nu. pr.F, int. 900 4.35 9.1 30 1-5 Comp. Ex. LiCoO₂ ALPO₄ Co:Al = 100:1 nu. pr.P, int. 900 4.35 9.1 25 1-6 Comp. Ex. LiCoO₂ Li₂CO₃ Co:Li:P = 100:1:1 —— P, int. 900 4.35 9.1 20 1-7 (NH₄)₂HPO₄ Comp. Ex. LiCoO₂ Li₃PO₄ Co:Li:P= 100:3:1 — — P, int. 900 4.35 9.1 15 1-8 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ Co:Li:P = 98:1:1 u. pr. P, u. 3004.35 8.6 35 1-9 (NH₄)₂HPO₄ Notes:- u.: uniform, nu.: nonuniform, pr.:present, ab.: absent, int.: interspersed

As is seen from the evaluation results, a capacity retention as well asa good initial capacity could be realized in Examples in which use wasmade of a positive electrode active material such that magnesium (Mg)was distributed uniformly, with a concentration gradient from the insidetoward the surface of the composite oxide particle, and that theparticle surfaces are coated in such a manner as to have sulfur (S),phosphorus (P) or the like in an interspersed state.

On the other hand, in Comparative Examples 1-1 to 1-3 in which thecoating material was absent, the capacity retention was lowered, moreconspicuously as the charging voltage of the cell was higher. Besides,in Comparative Examples 1-4 to 1-6 in which the distribution ofmagnesium (Mg) inside the oxide particles was nonuniform, a highcapacity retention could not be maintained, even if a concentrationgradient was present. Further, in the cases where the above-mentionedmetallic element M2 was absent, the capacity retention was very low,even if sulfur (S), phosphorus (P) or the like was present interspersedon the surfaces of the oxide particles.

In Examples form 2-1 to 2-9, coating material was varied and cellproperties with positive materials different in coating materials at thesurface of composite oxide are obtained.

Example 2-1

The same nonaqeuous electrolyte secondary cell as in Example 1-1 exceptAmmonium phosphate dibasic ((NH₄)₂HPO₄) with average diameter measuredby laser scattering method of 10 μm and with melting point of 190° C. isdeposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) instead of lithium carbonate anddiammonium hydrogenphosphate is prepared and evaluated in the samemanner as in example 1-1 with charging voltage of 4.35. The initialcapacity was 9.1 Wh and capacity retention was 85%.

Example 2-2

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptAmmonium sulphate ((NH₄)₂HSO₄) with average diameter measured by laserscattering method of 10 μm and with melting point of 513° C. isdeposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) is prepared. The initial capacity was9.1 Wh and capacity retention was 87%.

Example 2-3

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptAmmonium phosphate dibasic ((NH₄)₂HPO₄) with average diameter measuredby laser scattering method of 30 μm and with melting point of 190° C. isdeposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) is prepared. The initial capacity was9.1 Wh and capacity retention was 80%.

Example 2-4

The same nonaqeuous electrolyte secondary cell as in Example 2-1 isprepared except Lithium cobalt composite oxide(LiNi_(0.79)Co_(0.19)Al_(0.01)Mg_(0.01)O₂) is coated. The initialcapacity was 10.9 Wh and capacity retention was 81%.

Example 2-5

The same nonaqeuous electrolyte secondary cell as in Example 2-1 isprepared except Lithium cobalt composite oxide(LiNi_(0.49)CO_(0.19)Mn_(0.29)Al_(0.01)Mg_(0.01)O₂) is coated. Theinitial capacity was 9.5 Wh and capacity retention was 80%.

Example 2-6

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptphosphatic acid (H₃PO₄) with average diameter measured by laserscattering method of 10 μm and with melting point of 43° C. is depositedon lithium-cobalt composite oxide (LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) isprepared. The initial capacity was 9.1 Wh and capacity retention was53%.

Example 2-7

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptiron sulfate (Fe2(SO4)3) with average diameter measured by laserscattering method of 10 μm and with melting point of 480° C. isdeposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) is prepared. The initial capacity was8.9 Wh and capacity retention rate was 80%.

Example 2-8

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptammonium phosphate dibasic ((NH₄)₂HPO₄) with average diameter measuredby laser scattering method of 100 μm and with melting point of 190° C.is deposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) is prepared. The initial capacity was9.1 Wh and capacity retention was 58%.

Example 2-9

The same nonaqeuous electrolyte secondary cell as in Example 2-1 exceptammonium phosphate dibasic ((NH₄)₂HPO₄) with average diameter measuredby laser scattering method of 100 μm and with melting point of 837° C.is deposited on lithium-cobalt composite oxide(LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂) is prepared. The initial capacity was9.0 Wh and capacity retention was 60%.

The evaluation results are shown in Table 2 below. In Table 2, result ofComparative Example 1-1 is also shown for reference.

TABLE 2 Coating material Average Composite particle Melting Firing Volt-Initial Capacity oxide diameter point Addition temp. age capacityretention particle Material [μM] [° C.] amounts [° C.] [V] [Wh] [%]Example LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ (NH₄)₂HPO₄ 10 190 Co:P = 99:1900 4.35 9.1 85 2-1 Example (NH₄)₂HPO₄ 10 513 Co:S = 99:1 9.1 87 2-2Example (NH₄)₂HPO₄ 30 190 Co:P = 99:1 9.1 80 2-3 ExampleLiNi_(0.79)Co_(0.19)Al_(0.01)Mg_(0.01)O₂ (NH₄)₂HPO₄ 10 190 Ni + Co:P =99:1 10.9 81 2-4 ExampleLiNi_(0.49)Co_(0.19)MN_(0.29)Al_(0.02)Mg_(0.01)O₂ (NH₄)₂HPO₄ 10 190 Ni +Co + Mn:P = 99:1 9.5 80 2-5 Example LiC_(0.98)Al_(0.01)Mg_(0.01)O₂ H₃PO₄10 43 Co:P = 99:1 9.1 53 2-6 Example Fe₂(SO₄) 10 480 Co:S = 99:1 8.9 802-7 Example (NH₄)₂HPO₄ 100 190 Co:P = 99:1 9.1 58 2-8 Example (NH₄)₂HPO₄10 837 Co:P = 99:1 9.0 60 2-9 Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂— — — — 900 4.35 9.2 31 1-1

As is seen from the evaluation results, a capacity retention could berealized in Examples 2-1 to 2-5, in which the compound or the pyrolizedcompound containing phosphorus P or fluorine F has the melting point of80 to 600° C.

It can be presumed that the compound or the pyrolized compoundcontaining phosphorus P or fluorine becomes liquid when fired at 900° C.and coating uniformly the surface of the composite oxide. In theseExamples, initial capacities are maintained high because ammonium isevaporated and not remaining in the active materials.

In Examples 2-4 and 2-5, when using Lithium-nickel-cobalt compositeoxide or Lithium-nickel-cobalt-manganese composite oxide is used as acentral material of the positive electrode active material, positiveelectrode active material with a concentration gradient such that theconcentration of the metallic element M2 increases from the centertoward the surface of the composite oxide particle with good capacityretention is obtained.

As for Example 2-6, capacity retention is improved but improvement owingto the coating was not much. It is because phosphoric acid was dissolvedduring mechanochemical treatment and coating was not as effective asthose in Examples 2-1 to 2-5. This is caused because the melting pointof phosphoric acid is lower than the temperature of mechanochemicaltreatment.

As for Example 2-7, coating was preferable since the melting point ofthe compound falls within the range from 70 to 600° C. but decomposedmaterial remained on the surface of the positive electrode and initialdischarging capacity was slightly decreased because of this impuritywhich does not contribute to charge or discharge reaction.

In Example 2-8, coating material was not well mixed with composite oxidebecause of too large a diameter of coating material. Therefore volumeretention was improved but improvement owing to the coating was notmuch. It is because the melting point of coating material was 837° C.,being higher than 600° C. and near to the firing temperature, whichleads to the regional reaction of coating material with composite oxidebefore coating material melts and preferably coats the composite oxide,and disturbs preferable coating.

In Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3 to 14, ratio dand molar fraction ratio r were varied and cell properties weredetermined.

In these Examples, ratio d and molar fraction ratio r were obtained asfollows.

[Ratio d and Molar Fraction Ratio r]

Buffering solvent prepared to pH5.1 with citric acid and sodium citratewas added to 0.2 g of lithium-transition metal composite oxide. Mixturewas stirred up and sample was filtered with 0.2 μm filter every minutes.Mass or volume concentration of the principal transition metal M1, i.e.,Co and mass or volume concentration of the metallic element M2 i.e., Mg,Mn, Ni contained in each of the sample was measured by ICP-AES:Inductively Coupled Plasma Atomic Emission Spectrometry method [HORIBAJY238 ULTRACE] to obtain mass of M1 and M2 dissolved in 10 ml of buffersolvent. With the result, amounts [mol] of M1 and M2 were calculated.Ratio d and molar fraction ratio r were determined according to theformula (I) and (II).

ratio d(%)=[(mass of the principal transition metal M1+(mass of metallicelement M2)]/(mass of whole particles)  (I)

molar fraction r=(mass of metallic element M2)/[(mass of principaltransition metal M1+(mass of metallic element M2)]  (II)

The coating including M2 therein is most effective state for capacityretention and high temperature preservability where ratio d satisfies0.20≦r≦0.80, i.e., 10 to 100 nm depth from the surface. In followingExamples, molar fraction ratio r was varied with ratio d in the range of0.20≦r≦0.80 and cell properties of each cell were examined.

In the following Examples, distribution states of metallic element M2and element X were determined as follows.

[Distribution States of Metallic Element M2 and Element X]

By SEM/EDX Mg is examined to confirm whether Mg on the surface of theparticle is uniformly distributed or not and P is interspersed on thesurface or not. Particle was cut and element distribution along diameterwas measured by Auger Electron Spectroscopy to observe the continuousvariance of Mg in consistency.

Example 3-1

Positive electrode active material was prepared as follows.

The Precursor for firing in the same manner as in Example 1-1 wassubjected to a temperature rise at a rate of 3° C./min, and was held at900° C. for 3 hr, followed by slow cooling, to obtain alithium-transition metal composite oxide. The lithium-transition metalcomposite oxide had magnesium (Mg) uniformly distributed at the surfacesof the lithium-cobalt composite oxide particles. Besides, theconcentration of magnesium (Mg) was higher at the particle surfaces thanin the inside of the particles, and lithium phosphate (Li3PO4) wasinterspersed on the particle surfaces.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.02%, 0.05%were 0.32, 0.30 respectively. Molar fraction ratios r at ratio d=0.01%,0.10% were 0.46, 0.25 respectively.

The surface state of the lithium-transition metal composite oxide wasconfirmed by observation of the obtained powder under SEM/EDX. Upon theobservation of the surfaces of the lithium-transition metal compositeoxide, uniform distribution of magnesium (Mg) on the particle surfacesand interspersing of phosphorus on the particle surfaces were confirmed.The particle was subjected to measurement of powder X-ray diffractionpattern by use of CuKα, a diffraction peak corresponding to Li₃PO₄ wasconfirmed in addition to a diffraction peak corresponding to LiCoO₂having a laminar rock salt structure. In addition, magnesiumconcentration was confirmed by cutting a section of thelithium-transition metal composite oxide, and measuring the elementdistribution in the radial direction by Auger electron spectroscopy.Upon the measurement of the element distribution in the section of thelithium-transition metal composite oxide, the magnesium concentrationwas confirmed to be varying continuously from the surface toward theinside of the particle.

By using as a positive electrode active material the lithium-transitionmetal composite oxide obtained as above, a nonaqueous electrolytesecondary cell was manufactured according to the same method as inExample 1-1.

The cell was subjected to evaluations of initial capacity, capacityretention and high-temperature storage property. The high-temperaturestorage property was determined as follows.

The cell manufactured as above was subjected to charging at a chargingcurrent of 1.5 A up to a charging voltage of 4.35 V in an environment ofan ambient temperature of 45° C. Immediately thereafter, the cell wassubjected to discharging at a discharging current of 2.0 A, and thedischarging was finished when the cell voltage was lowered to 3.0 V.Then the cell was subjected to high-temperature storage by being left inan environment of an ambient temperature of 60° C. for 300 hr. Afterthat, the discharge capacity after high-temperature storage was measuredby discharging at 0.2° C. With initial capacity and discharge capacityafter high-temperature storage, the high temperature capacity retentionrate, i.e., high-temperature storage preservability was obtainedaccording to the following formula. High temperature capacity retentionrate [%]=(discharge capacity after high-temperature storage/initialcapacity)×100

Example 3-2

Nonaqueous electrolyte secondary cell was prepared in the same manner asin Example 3-1, and subjected to evaluation of initial capacity,capacity retention and high-temperature storage retention property inthe same manner as in Example 3-1 except that charging voltage was 4.2V.

Example 3-3

Nonaqueous electrolyte secondary cell was prepared in the same manner asin Example 3-1, and subjected to evaluation of initial capacity,capacity retention and high-temperature storage retention property inthe same manner as in Example 3-1 except that charging voltage was 4.5V.

Example 3-4

Positive electrode active material was prepared in the same manner as inExample 3-1 except that the temperature of second firing was set to 950°C. and the time of the second firing was 30 minutes. Molar fractionratios r at ratio d=0.02%, 0.05% were 0.22, 0.21 respectively. Molarfraction ratios r at ratio d=0.01%, 0.10% were 0.38, 0.16 respectively.Nonaqueous electrolyte secondary cell was prepared in the same manner asin Example 3-1 and was subjected to evaluation of initial capacity,capacity retention and high-temperature storage retention property inthe same manner as in Example 3-1 except that charging voltage was 4.5V.

Example 3-5

Positive electrode active material was prepared in the same manner as inExample 3-1 except that composite oxide as base material wasLiCo0.95Al0.01Mg0.04O2. Molar fraction ratios r at ratio d=0.02%, 0.05%were 0.73, 0.52 respectively. Molar fraction ratios r at ratio d=0.01%,0.10% were 0.86, 0.44 respectively. Nonaqueous electrolyte secondarycell was prepared in the same manner as in Example 3-1 and was subjectedto evaluation of initial capacity, capacity retention andhigh-temperature storage retention property in the same manner as inExample 3-1.

Example 3-6

Positive electrode active material was prepared in the same manner as inExample 3-1 except that composite oxide as base material wasLiCo0.97Al0.01Mg0.02O2. Molar fraction ratios r at ratio d=0.02%, 0.05%were 0.31, 0.31 respectively. Molar fraction ratios r at ratio d=0.01%,0.10% were 0.56, 0.25 respectively. Nonaqueous electrolyte secondarycell was prepared in the same manner as in Example 3-1 and was subjectedto evaluation of initial capacity, capacity retention andhigh-temperature storage retention property in the same manner as inExample 3-1.

Example 3-7

Positive electrode active material was prepared in the same manner as inExample 3-1 except that composite oxide as base material was LiCoO₃ andmixed with Lithium carbonate Li₂CO₃, Magnesium carbonate MgCO₃, Ammoniumphosphate dibasic NH₄H₂PO₄ in ratios shown in table 3 and 4. Molarfraction ratios r at ratio d=0.02%, 0.05% were 0.46, 0.40 respectively.Molar fraction ratios r at ratio d=0.01%, 0.10% were 0.55, 0.44respectively. Nonaqueous electrolyte secondary cell was prepared in thesame manner as in Example 3-1 and was subjected to evaluation of initialcapacity, capacity retention and high-temperature storage retentionproperty in the same manner as in Example 3-1.

Example 3-8

Positive electrode active material was prepared in the same manner as inExample 3-1 except that LiCoO2 was used as Lithium Cobalt compositeoxide for base material and was coated with coating material of nickelhydroxide and manganese phosphate. On coating, materials were preparedand mixed so that molar fraction ratios r (Ni+Mn/Ni+Mn+Co) at ratiod=0.02%, 0.05% were 0.35, 0.34 respectively and molar fraction ratios rat ratio d=0.01%, 0.10% were 0.56, 0.25 respectively. Nonaqueouselectrolyte secondary cell was prepared in the same manner as in Example3-1 and was subjected to evaluation of initial capacity, capacityretention and high-temperature preservability in the same manner as inExample 3-1.

Comparative Example 3-1

Composite oxide LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂ without coating was usedas positive electrode active material. Molar fraction ratios r at ratiod=0.02%, 0.05% were 0.01, 0.01 respectively. Molar fraction ratios r atratio d=0.01%, 0.10% were 0.01, 0.01 respectively. Nonaqueouselectrolyte secondary cell was prepared in the same manner as in Example3-1 and was subjected to evaluations of initial capacity, capacityretention and high-temperature preservability in the same manner as inExample 3-1.

Comparative Example 3-2

Positive electrode active material was prepared in the same manner as inExample 3-1 except that LiCoO2 was used for Lithium Cobalt compositeoxide as a base material and mixed with Lithium carbonate LiCoO3,Magnesium carbonate MgCo3, and ammonium phosphate dibasic NH₄H₂PO₄ in amolar ratio of Co:Li:Mg:P=100:1:0.5:1. Molar fraction ratios r at ratiod=0.02%, 0.05% were 0.18, 0.10 respectively. Molar fraction ratios r atratio d=0.01%, 0.10% were 0.25, 0.08 respectively. Nonaqueouselectrolyte secondary cell was prepared in the same manner as in Example3-1 and subjected to evaluation of initial capacity, capacity retentionand high-temperature preservability in the same manner as in Example3-1.

Comparative Example 3-3

Positive electrode active material was prepared in the same manner as inExample 3-1 except that Lithium Cobalt composite oxideLiCO_(0.98)Al_(0.01)Mg_(0.01)O₂, Lithium carbonate LiCoO3, Magnesiumcarbonate MgCo3, and ammonium phosphate dibasic NH₄H₂PO₄ were mixed in amolar ratio of Co:Li:Mg:P=100:1:1:4. Molar fraction ratios r at ratiod=0.02%, 0.05% were 0.82, 0.83 respectively. Molar fraction ratios r atratio d=0.01%, 0.10% were 0.80, 0.85 respectively. Nonaqueouselectrolyte secondary cell was prepared in the same manner as in Example3-1 and subjected to evaluation of initial capacity, capacity retentionand high-temperature preservability in the same manner as in Example3-1.

Comparative Example 3-4

Positive electrode active material was prepared in the same manner as inExample 3-1 except that the temperature of second firing was set to 950°C. and the time of the second firing as 30 minutes. Molar fractionratios r at ratio d=0.02%, 0.05% were 0.22, 0.21 respectively. Molarfraction ratios r at ratio d=0.01%, 0.10% were 0.38, 0.16 respectively.Nonaqueous electrolyte secondary cell was prepared in the same manner asin Example 3-1 and subjected to evaluation of initial capacity, capacityretention and high-temperature preservability in the same manner as inExample 3-1.

Comparative Example 3-5

Positive electrode active material was prepared in the same manner as inExample 3-1 except that except that LiCo0.95Al0.01Mg0.04O2 was used forLithium Cobalt composite oxide as a base material Molar fraction ratiosr at ratio d=0.02%, 0.05% were 0.73, 0.52 respectively. Molar fractionratios r at ratio d=0.01%, 0.10% were 0.86, 0.44 respectively.Nonaqueous electrolyte secondary cell was prepared in the same manner asin Example 3-1 and subjected to evaluation of initial capacity, capacityretention and high-temperature preservability in the same manner as inExample 3-1.

Comparative Example 3-6

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Comparative Example 3-5. The sell was subjected toevaluation of initial capacity, capacity retention and high-temperaturepreservability in the same manner as in Example 3-1 except that the cellvoltage upon charging was 4.2 V.

Comparative Example 3-7

A nonaqueous electrolyte secondary cell was manufactured in the samemanner as in Comparative Example 3-5. The sell was subjected toevaluation of initial capacity, capacity retention and high-temperaturepreservability in the same manner as in Example 3-1 except that the cellvoltage upon charging was 4.5 V.

Comparative Example 3-8

Positive electrode active material was prepared in the same manner as inExample 3-1 except that second firing procedure was omitted. Molarfraction ratios r at ratio d=0.02%, 0.05% were 0.80, 0.81 respectively.Molar fraction ratios r at ratio d=0.01%, 0.10% were 0.82, 0.79respectively. Nonaqueous electrolyte secondary cell was prepared in thesame manner as in Example 3-1 and subjected to evaluation of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-1.

Comparative Example 3-9

Positive electrode active material was prepared in the same manner as inExample 3-1 except that mechano-chemical treatment was done for 15minutes. Molar fraction ratios r at ratio d=0.02%, 0.05% were 0.21, 0.16respectively. Molar fraction ratios r at ratio d=0.01%, 0.10% were 0.31,0.14 respectively. Nonaqueous electrolyte secondary cell was prepared inthe same as in Example 3-1 and subjected to evaluation of initialcapacity, capacity retention and high-temperature preservability in thesame way as in Example 3-1.

Comparative Example 3-10

Positive electrode active material was prepared in the same manner as inExample 3-1 except that LiCoO2 was used as Lithium Cobalt compositeoxide for a base material and was coated with coating material of nickelhydroxide and manganese phosphate. On coating, materials were preparedand mixed so that molar ratio of Ni:Co:Mn=1:1:1 in the whole positiveelectrode active material particle and that molar fraction ratios r(Ni+Mn/Ni+Mn+Co) at ratio d=0.02%, 0.05% were 0.25, 0.17 respectivelyand molar fraction ratios r at ratio d=0.01%, 0.10% were 0.30, 0.15respectively. Nonaqueous electrolyte secondary cell was prepared in thesame manner as in Example 3-1 and subjected to evaluation of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-1.

The structure of the positive electrode active material of thenonaqeuous electrolyte secondary cell and evaluation results of Examples3-1 to 3-8 and Comparative Examples 3-1 to 3-10 are shown in Tables 3and 4 below.

TABLE 3 Distr. States Mg Coating Addition conc. Base material materialamounts Mg gradient Surface Example LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-1 ExampleLiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-2 Example LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-3 ExampleLiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-4 Example LiCo_(0.95)Al_(0.01)Mg_(0.04)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-5 ExampleLiCo_(0.98)Al_(0.01)Mg_(0.02)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-6 Example LiCoO₂ Li₂CO₃+ MgCO₃ + NH₄H₂PO₄ Co:Li:Mg:P = u.pr. P, int. 3-7 100:1:2:1 Example LiCoO₂ Ni(OH)₂, MnPO₄ Ni:Co:Mn = 5:2:3Ni, pr. P, int. 3-8 Mn, u. Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ — —u. ab. X, ab. 3-1 Comp. Ex. LiCoO₂ Li₂CO₃ + MgCO₃₊NH₄H₂PO₄ Co:Li:Mg:P =nu. pr. P, int. 3-2 100:1:0.5:1 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:4 u.pr. P, int. 3-3 Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-4 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-5 Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-6 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-7 Comp. Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 u. pr. P, int. 3-8 Comp. Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 3-9 Comp. Ex. LiCoO₂ Ni(OH)₂, MnPO₄ Ni:Co:Mn = 1:1:1 Ni, pr.P, int. 3-10 Mn, u. Initial High- Molar Molar discharge Capacitytemperature fraction r fraction r Voltage capacity retentionpreserveability (d = 0.02%) (d = 0.05%) [V] [Wh] [%] [%] Example 0.320.30 4.35 9.1 82 90 3-1 Example 0.32 0.30 4.20 8.0 85 94 3-2 Example0.32 0.30 4.50 10.0  68 80 3-3 Example 0.22 0.21 4.35 9.3 81 90 3-4Example 0.73 0.52 4.35 8.8 87 94 3-5 Example 0.31 0.30 4.35 9.4 84 913-6 Example 0.46 0.40 4.35 8.7 77 86 3-7 Example 0.35 0.34 4.35 8.5 7587 3-8 Comp. Ex. 0.01 0.01 4.35 9.2 31 75 3-1 Comp. Ex. 0.18 0.10 4.359.1 32 76 3-2 Comp. Ex. 0.82 0.83 4.35 8.1 65 79 3-3 Comp. Ex. 0.81 0.754.35 9.2 32 77 3-4 Comp. Ex. 0.62 0.71 4.35 9.0 36 74 3-5 Comp. Ex. 0.620.71 4.20 8.0 39 82 3-6 Comp. Ex. 0.62 0.71 4.50 9.9 21 68 3-7 Comp. Ex.0.80 0.81 4.35 9.2 30 76 3-8 Comp. Ex. 0.21 0.16 4.35 9.2 38 75 3-9Comp. Ex. 0.25 0.17 4.35 8.6 50 69 3-10 Notes:- u.: uniform, nu.:nonuniform, pr.: present, ab.: absent, int.: interspersed

TABLE 4 Coating Addition Base material material amounts Example 3-1LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1Example 3-2 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P =98:1:1 Example 3-3 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄Co:Li:P = 98:1:1 Example 3-4 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 Example 3-5 LiCo_(0.95)Al_(0.01)Mg_(0.04)O₂Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 Example 3-6LiCo_(0.98)Al_(0.01)Mg_(0.02)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1Example 3-7 LiCoO₂ Li₂CO₃ + MgCO₃ + NH₄H₂PO₄ Co:Li:Mg:P = 100:1:2:1Example 3-8 LiCoO₂ Ni(OH)₂, MnPO₄ Ni:Co:Mn = 5:2:3 Comp. Ex. 3-1LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ — — Comp. Ex. 3-2 LiCoO₂ Li₂CO₃ +MgCO₃ + NH₄H₂PO₄ Co:Li:Mg:P = 100:1:0.5:1 Comp. Ex. 3-3LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:4 Comp.Ex. 3-4 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P =98:1:1 Comp. Ex. 3-5 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄Co:Li:P = 98:1:1 Comp. Ex. 3-6 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ +NH₄H₂PO₄ Co:Li:P = 98:1:1 Comp. Ex. 3-7 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 Comp. Ex. 3-8LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 Comp.Ex. 3-9 LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P =98:1:1 Comp. Ex. 3-10 LiCoO₂ Ni(OH)₂, MnPO₄ Ni:Co:Mn = 1:1:1 Distr.States Mg Molar Molar conc. fraction r fraction r Mg gradient Surface (d= 0.01%) (d = 0.10%) Voltage [V] Example 3-1 u. pr. P, int. 0.46 0.254.35 Example 3-2 u. pr. P, int. 0.46 0.25 4.20 Example 3-3 u. pr. P,int. 0.46 0.25 4.50 Example 3-4 u. pr. P, int. 0.38 0.16 4.35 Example3-5 u. pr. P, int. 0.86 0.44 4.35 Example 3-6 u. pr. P, int. 0.56 0.254.35 Example 3-7 u. pr. P, int. 0.55 0.44 4.35 Example 3-8 Ni, pr. P,int. 0.52 0.33 4.35 Mn, u. Comp. Ex. 3-1 u. ab. X, ab. 0.01 0.01 4.35Comp. Ex. 3-2 un. pr. P, int. 0.25 0.08 4.35 Comp. Ex. 3-3 u. pr. P,int. 0.80 0.85 4.35 Comp. Ex. 3-4 u. pr. P, int. 0.81 0.70 4.35 Comp.Ex. 3-5 u. pr. P, int. 0.55 0.75 4.35 Comp. Ex. 3-6 u. pr. P, int. 0.550.75 4.20 Comp. Ex. 3-7 u. pr. P, int. 0.55 0.75 4.50 Comp. Ex. 3-8 u.pr. P, int. 0.82 0.79 4.35 Comp. Ex. 3-9 u. pr. P, int. 0.31 0.14 4.35Comp. Ex. 3-10 Ni, pr. P, int. 0.30 0.15 4.35 Mn, u. Notes:- u.:uniform, nu.: nonuniform, pr.: present, ab.: absent, int.: interspersed

As is seen from the evaluation results, good capacity retention andhigh-temperature preservability could be realized with controllingdecline of initial capacity in Examples 3-1 to 3-8. On the other hand,in Comparative Examples 3-1 to 3-10 these effects were not attained.

Molar fraction ratios r of the positive electrode active materials ofExamples 3-1 to 3-8 fall within the range 0.20≦r≦0.80 in the in therange that ratio d satisfies 0.02%≦d≦0.05% were 0.21, 0.16 respectively.And they shows the tendency that molar ratio r decreases from thesurface into the depth direction in the range where ratio d satisfies0.020%≦d≦0.050%.

Molar fraction ratios r of the positive electrode active materials ofComparative Examples 3-1 to 3-2, 3-4, 3-9 to 3-10 is constant ordecreases along the depth direction from the surface where ratio d(%)satisfies 0.02%≦d≦0.05% in a certain range of depth, i.e., 10 to 100 nmfrom the surface. But molar fraction ratio r does not fall in the rangeof 0.20≦r≦0.80

Molar fraction ratio r of Comparative Example 3-1 was not in the rangeof 0.20≦r≦0.80 because coating material was not used. Molar fractionratio r of Comparative Example 3-2 was not in the range of 0.20≦r≦0.80because mixing volumes of base material and coating materials are notappropriate. Molar fraction ratio r of Comparative Example 3-4 was notin the range of 0.20≦r≦0.80 because second firing temperature was 750°C. Molar fraction ratio r of Comparative Example 3-9 was not in therange of 0.20≦r≦0.80 because time of mechano-chemical procedure was 15minutes, which was too short in comparison with Example 3-1. Molarfraction ratio r of Comparative Example 3-10 was not in the range of0.20≦r≦0.80 because mixing volumes of base material and coatingmaterials are not appropriate.

Molar fraction ratios r of the positive electrode active materials ofComparative Examples 3-3 and 3-8 fall out of the range 0.20≦r≦0.80 whereratio d (%) satisfies 0.02%≦d≦0.05%. And they show the tendency thatmolar ratio r increases from the surface into the depth direction in therange where ratio d satisfies 0.020%≦d≦0.050% in a certain range ofdepth, i.e., 10 to 100 nm from the surface.

Molar fraction ratios r of the positive electrode active materials ofComparative Examples 3-3 fall out of the range 0.20≦r≦0.80 becausemixing volumes of base material and coating materials are notappropriate. Molar fraction ratios r of the positive electrode activematerials of Comparative Examples 3-8 fall out of the range 0.20≦r≦0.80because second firing procedure is not done.

Molar fraction ratios r of the positive electrode active materials ofComparative Examples 3-5 to 3-7 fall within the range 0.20≦r≦0.80. Butthey shows the tendency that molar fraction ratio r increase from thesurface into the depth direction in the range where ratio d satisfies0.020%≦d≦0.050% in a certain range of depth, i.e., 10 to 100 nm from thesurface.

Molar fraction ratios r of the positive electrode active materials ofComparative Examples 3-5 to 3-7 increase because temperature of thesecond firing procedure is 850° C.

As is shown above, good capacity retention and high-temperaturepreservability could be realized with controlling decline of initialcapacity when Molar fraction ratios r fall within the range 0.20≦r≦0.80in the range where ratio d satisfies 0.020%≦d≦0.050% in a certain rangeof depth from the surface.

As for preparing method of positive electrode active material, metallicelement M2 is preferably drawn from the basic material to the surface asin Examples 3-1 to 3-6. The preparing method makes the processes simple,and the prepared material has more uniform distribution at the surfaceand well retains the structure, which improves capacity retention andhigh-temperature preservability.

As is seen from the evaluation results in table 4, positive electrodeactive material with ratio d out of the range 0.020%≦d≦0.050% does notalways improve capacity retention or high-temperature preservabilityeven molar fraction rate r is in the range of 0.20≦r≦0.80.

As for the analyzing method of the surface of the positive electrodeactive material, XPS: X-ray Photoelectron Spectroscopy and TOF-SIMS:Time-Of-Flight Secondary Ion Mass Spectroscopy has been used heretofore.Table 3 shows measured molar fraction ratio r where ratio dcorresponding to the depth range along the direction of the depthdetected by these method was 0.010%, which corresponding to the area ofa few nm depth from the surface, and where ratio d corresponding to thedepth range along the direction of the depth detected by this method was0.100%, which corresponds to the area of more than 100 nm depth from thesurface.

Examples 3-9

Nonaqueous Electrolyte secondary battery prepared as in Example 3-4 wasbroken down and positive electrode current collector was peeled from theelectrode and binder was removed by immersing in NMP from the positiveelectrode active material, conductive agent was burned to obtainpositive electrode active material. Molar fraction rate r at the ratiod=0.02% and 0.05% were 0.29 and 0.22 respectively.

Examples 3-10

Nonaqueous Electrolyte secondary battery prepared as in Example 3-5 wasbroken down and positive electrode current collector was peeled from theelectrode and binder was removed by immersing in NMP from the positiveelectrode active material, conductive agent was burned to obtainpositive electrode active material. Molar fraction rate r at the ratiod=0.02% and 0.05% were 0.79 and 0.53 respectively.

The structure of the positive electrode active material of thenonaqeuous electrolyte secondary cell and evaluation results of Examples3-9 and 3-10 are shown in Table 5 below.

Distr. States Mg Molar Molar Coating Addition conc. fraction r fractionr Base material material amounts Mg gradient Surface (d = 0.02%) (d =0.05%) Example LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P= 98:1:1 u. pr. P, int. 0.29 0.22 3-9 ExampleLiCo_(0.95)Al_(0.01)Mg_(0.01)O₂ Li₂CO₃ + NH₄H₂PO₄ Co:Li:P = 98:1:1 u.pr. P, int. 0.79 0.53 3-10 Notes:- u.: uniform, nu.: nonuniform, pr.:present, ab.: absent, int.: interspersed

Table 5 shows molar fraction rates r at the ratio d between 0.02% and0.05% fall within the range of 0.20<r<0.80 when positive electrodeactive material was taken out form Nonaqueous electrolyte secondarycells.

Example 3-11

Positive electrode active material was prepared as follows.

Lithium Cobalt oxide LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂ as the same as inExample 3-1 with average diameter of 13 μm measured by laser scatteringmethod and ammonium phosphate dibse NH₄H₂PO₄ powdered by jet-mill intoaverage diameter of 6 μm measured by laser scattering method were mixedin atomic ratio of Co:P=99:1.

The mixture was treated by mechano-chemical device for 1 hr. to depositeammonium phosphate dibsse on the surface of Lithium Cobalt Oxide toobtain precursor before firing. The precursor was subjected to atemperature rise at a rate of 3° C./min, and was held at 900° C. for 3hr, followed by slow cooling, to obtain a lithium-transition metalcomposite oxide. The lithium-transition metal composite oxide hadmagnesium (Mg) uniformly distributed at the surfaces of thelithium-cobalt composite oxide particles. Besides, the concentration ofmagnesium (Mg) was higher at the particle surfaces than in the inside ofthe particles, and lithium phosphate (Li₃PO₄) was interspersed on theparticle surfaces.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.01%, 0.015%,0.02%, 0.05% were 0.82, 0.73, 0.62, and 0.40 respectively.

The surface state of the obtained material was confirmed by observationof the obtained powder under SEM/EDX. Upon the observation uniformdistribution of magnesium (Mg) on the particle surfaces andinterspersing of phosphorus on the particle surfaces were confirmed. Theparticle was subjected to measurement of powder X-ray diffractionpattern by use of CuKα, a diffraction peak corresponding to Li₃PO₄ wasconfirmed in addition to a diffraction peak corresponding to LiCoO₂having a laminar rock salt structure. In addition, magnesiumconcentration was confirmed by cutting a section of thelithium-transition metal composite oxide, and measuring the elementdistribution in the radial direction by Auger electron spectroscopy.Upon the measurement of the element distribution in the section of theparticle, the magnesium concentration was confirmed to be varyingcontinuously from the surface toward the inside of the particle.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particle s obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 3-1. and the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-1.

Example 3-12

Positive electrode active material was prepared in the same manner as inExample 3-11 except that Lithium Cobalt oxideLiCO_(0.98)Al_(0.01)Mg_(0.01)O₂ with average diameter of 6 μm measuredby laser scattering method and ammonium phosphate dibsse NH₄H₂PO₄powdered by jet-mill into average diameter of 6 μm measured by laserscattering method were mixed in atomic ratio of Co:P=98.8:1.2.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.01%, 0.015%,0.02%, 0.05% were 0.92, 0.85, 0.80, and 0.65 respectively.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particle s obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 1-1. And the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-11.

Example 3-13

Lithium Cobalt oxide LiCO_(0.98)Al_(0.01)Mg_(0.01)O₂ with averagediameter of 6 μm measured by laser scattering method and ammoniumsulfate 4 2 4 powdered by jet-mill into average diameter of 3 μmmeasured by laser scattering method were mixed in atomic ratio ofCo:S=99:1. The mixture was treated by Planetary Mixer for 30 minutes todeposite ammonium sulfate on the surface of Lithium Cobalt oxide.Positive electrode active material was prepared in the same manner as inExample 3-11 except mentioned. Molar fraction ratios r at ratio d=0.01%,0.015%, 0.02%, 0.05% were 0.80, 0.71, 0.58, and 0.38 respectively.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particles obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 3-11. and the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-11.

Example 3-14

Positive electrode active material was prepared in the same manner as inExample 3-11 except that Lithium Cobalt oxide with average diameter of100 μm measured by laser scattering method and ammonium phosphate dibsseNH₄H₂PO₄ were mixed in atomic ratio of Co:P=99:1.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.01%, 0.015%,0.02%, 0.05% were 0.62, 0.53, 0.44, and 0.25 respectively.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particle s obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 3-11. and the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-11.

Comparative Example 3-11

Positive electrode active material was prepared in the same manner as inExample 3-11 except that Lithium Cobalt oxide with average diameter of 6μm measured by laser scattering method and ammonium phosphate dibsaseNH₄H₂PO₄ were mixed in atomic ratio of Co:P=95:5.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.01%, 0.015%,0.02%, 0.05% were 0.98, 0.95, 0.92, and 0.85 respectively.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particle s obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 3-11. and the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-11.

Comparative Example 3-12

Positive electrode active material was prepared in the same manner as inExample 3-11 except that Lithium Cobalt oxide with average diameter of 6μm measured by laser scattering method and ammonium sulfate (NH4)2SO4having average diameter of 6 μm measured by laser scattering method weremixed in atomic ratio of Co:S=99:1.

Incidentally, the surface concentration gradient of Magnesium Mg wasconfirmed in detail. Molar fraction ratios r at ratio d=0.01%, 0.015%,0.02%, 0.05% were 0.33, 0.25, 0.20, and 0.15 respectively.

By using as a positive electrode active material the lithium-transitionmetal composite oxide particle s obtained as above, a nonaqueouselectrolyte secondary cell was manufactured in the same manner as inExample 3-13. and the cell was subjected to evaluations of initialcapacity, capacity retention and high-temperature preservability in thesame manner as in Example 3-13.

The structure of the positive electrode active material of thenonaqeuous electrolyte secondary cell and evaluation results of Examples3-11 to 3-14, Comparative Examples 3-11 to 3-12 are shown in Table 6below.

TABLE 6 Distr. States Molar Coating Addition Mg conc. fraction r Basematerial material amounts Mg gradient Surface (d = 0.01%) Ex.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ NH₄H₂PO₄ Co:P = 99:1 u. pr. P, int. 0.823-11 Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ NH₄H₂PO₄ Co:P = 98.8:1.2 u. pr.P, int. 0.92 3-12 Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ (NH₄)₂SO₄ Co:S =99:1 u. pr. S, int. 0.80 3-13 Ex. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂NH₄H₂PO₄ Co:P = 99:1 u. pr. P, int. 0.62 3-14 Comp.LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ NH₄H₂PO₄ Co:P = 95:5 u. pr. P, int. 0.98Ex 3-11 Comp. LiCo_(0.98)Al_(0.01)Mg_(0.01)O₂ (NH₄)₂SO₄ Co:S = 99:1 u.pr. S, int. 0.33 Ex. 3-12 Initial High- Molar Molar Molar dischargeCapacity temperature fraction r fraction r fraction r Voltage capacityretention preservability (d = 0.015%) (d = 0.02%) (d = 0.05%) [V] [Wh][%] [%] Ex. 0.73 0.62 0.40 4.35 10.00 90 95 3-11 Ex. 0.85 0.80 0.65 4.3510.10 92 96 3-12 Ex. 0.71 0.58 0.38 4.35 10.00 90 96 3-13 Ex. 0.53 0.440.25 4.35 9.80 72 83 3-14 Comp. 0.95 0.92 0.85 4.35 8.70 85 90 Ex 3-11Comp. 0.25 0.20 0.15 4.35 10.00 60 65 Ex. 3-12 Notes:- u.: uniform, nu.:nonuniform, pr.: present, ab.: absent, int.: interspersed

As is seen from the evaluation results shown in table 6, molar fractionratios r of the positive electrode active material of Examples 3-11,3-12, 3-13 fall within the range 0.20≦r≦0.80 in the range that ratio dsatisfies 0.02%≦d≦0.05%. And at the same time, molar fraction ratios rfall within the range 0.55≦r<1.0 in the range that ratio d satisfies0.01%≦d<0.02%. These Examples perform very improved retention capacityand high-temperature preservability with controlling their decline indischarge capacity.

Molar fraction ratio r of the positive electrode active material ofExamples 3-14 falls within the range 0.20≦r≦0.80 in the range that ratiod satisfies 0.02%≦d≦0.05%. But molar fraction ratios r do not fallwithin the range 0.55≦r<1.0 in the range that ratio d satisfies0.01%≦d<0.02%. Example 3-14 could not attain highly improved retentioncapacity or high-temperature preservability. It is because of theaverage diameter of coating material ammonium phosphate dibase being 100μm, which disturbs ammonium phosphate dibase from well mixed state tomake good coating state on the surface of the base material.

Molar fraction ratio r of the positive electrode active material ofComparative Example 3-11 falls within the range 0.55≦r<1.0 in the rangethat ratio d satisfies 0.01%≦d<0.02%. But molar fraction ratios r fallout of the range 0.20≦r≦0.80 in the range that ratio d satisfies0.02%≦d≦0.05%. This is because too much coating materials make too thincoatings, which decreases the positive electrode active materialscontributing to charge-discharge capacity. Consequently, initialdischarge capacity by Comparative Example 3-11 is small.

Molar fraction ratio r of the positive electrode active material ofComparative Example 3-12 falls out of the range 0.55≦r<1.0 in the rangethat ratio d satisfies 0.01%≦d<0.02%. This is because coating materialswere not well mixed with base material and could not make good coatings.Therefore, good improvement in retention capacity and high-temperaturepreservability could not be attained by Comparative Example 3-12.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

The application is claimed as follows:
 1. A positive electrode activematerial prepared by: mixing a lithium-containing compound, a compoundcontaining a transition metal to be involved in a solid solution, and acompound containing a metallic element M2 different from the transitionmetal, and firing the mixture to form composite oxide particles;depositing a compound containing at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) on surfaces of thecomposite oxide particles; and firing the composite oxide particles withthe compound containing at least one element selected from among sulfur(S), phosphorus (P) and fluorine (F) deposited thereon; whereby each ofthe composite oxide particles is provided with a concentration gradientsuch that the concentration of the metallic element M2 increases fromthe center toward the surface of the composite oxide particle, and atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) is made present in the form of being aggregated at thesurfaces of the composite oxide particles.
 2. The positive electrodeactive material according to claim 1, wherein the transition metal is atleast selected from among nickel (Ni), cobalt (Co), manganese (Mn) andiron (Fe).
 3. The positive electrode active material according to claim1, wherein the metallic element M2 is at least one element selected fromamong manganese (Mn), magnesium (Mg), aluminum (Al), nickel (Ni), boron(B), titanium (Ti), cobalt (Co) and iron (Fe).
 4. The positive electrodeactive material according to claim 1, wherein the firing of thecomposite oxide particles with the compound containing at least oneelement selected from among sulfur (S), phosphorus (P) and fluorine (F)deposited thereon is conducted in the co-existence of alithium-containing compound.
 5. The positive electrode active materialaccording to claim 1, wherein the compound containing at least oneelement selected from among sulfur (S), phosphorus (P) and fluorine (F)or the thermally decomposed product of said compound has a melting pointof 70° C. or more and 600° C. or less.
 6. The positive active materialaccording to claim 1, wherein the compound containing at least oneelement selected from among sulfur (S), phosphorus (P) and fluorine (F)or the thermally decomposed product of said compound preferably has anaverage diameter of 30 μm or less.
 7. A positive electrode comprising apositive electrode material which is prepared by: mixing alithium-containing compound, a compound containing a transition metal tobe put into a solid solution, and a compound containing a metallicelement M2 different from the transition metal, and firing the mixtureto form composite oxide particles; depositing a compound containing atleast one element selected from among sulfur (S), phosphorus (P) andfluorine (F) on surfaces of the composite oxide particles; and firingthe composite oxide particles with the compound containing at least oneelement selected from among sulfur (S), phosphorus (P) and fluorine (F)deposited thereon; whereby each of the composite oxide particles isprovided with a concentration gradient such that the concentration ofthe metallic element M2 increases from the center toward the surface ofthe composite oxide particle, and at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) is made present in theform of being aggregated at the surfaces of the composite oxideparticles.
 8. A nonaqueous electrolyte cell comprising a positiveelectrode, a negative electrode, and an electrolyte, wherein thepositive electrode include a positive electrode active material which isprepared by: mixing a lithium-containing compound, a compound containinga transition metal to be put into a solid solution, and a compoundcontaining a metallic element M2 different from the transition metal,and firing the mixture to form composite oxide particles; depositing acompound containing at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) on surfaces of the composite oxideparticles; and firing the composite oxide particles with the compoundcontaining at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) deposited thereon; whereby each of thecomposite oxide particles is provided with a concentration gradient suchthat the concentration of the metallic element M2 increases from thecenter toward the surface of the composite oxide particle, and at leastone element selected from among sulfur (S), phosphorus (P) and fluorine(F) is made present in the form of being aggregated at the surfaces ofthe composite oxide particles.
 9. A method of preparing a positiveelectrode active material comprising: mixing a lithium-containingcompound, a compound containing a transition metal to be put into asolid solution, and a compound containing a metallic element M2different from the transition metal, and firing the mixture to formcomposite oxide particles; depositing a compound containing at least oneelement selected from among sulfur (S), phosphorus (P) and fluorine (F)on surfaces of the composite oxide particles; and firing the compositeoxide particles with the compound containing at least one elementselected from among sulfur (S), phosphorus (P) and fluorine (F)deposited thereon; whereby each of the composite oxide particles isprovided with a concentration gradient such that the concentration ofthe metallic element M2 increases from the center toward the surface ofthe composite oxide particle, and at least one element selected fromamong sulfur (S), phosphorus (P) and fluorine (F) is made present in theform of being aggregated at the surfaces of the composite oxideparticles.
 10. The method of preparing a positive electrode activematerial according claim 9; wherein during firing, the compoundcontaining at least one element selected from among sulfur (S),phosphorus (P) and fluorine (F) is melted or thermally decomposed andmelted, and the melted compound uniformly coats the surface of thecomposite oxide particles.
 11. The method of preparing a positiveelectrode active material according claim 10; wherein cation from thedeposited compound containing at least one element selected from amongsulfur (S), phosphorus (P) and fluorine (F) is eliminated during firing,and anion from the deposited compound containing at least one elementselected from among sulfur (S), phosphorus (P) and fluorine (F) reactswith an element included in the composite oxide particle.